feat: add observability metrics

integrations/jikan/client.go

@@ -16,6 +16,7 @@ import (
 	"time"
 
 	"mal/internal/db"
+	"mal/internal/observability"
 
 	"golang.org/x/sync/singleflight"
 )
@@ -29,6 +30,7 @@ type Client struct {
 	lastReqTime time.Time // rate limiting: last request timestamp
 	sf          singleflight.Group
 	refreshSem  chan struct{}
+	metrics     *observability.Metrics
 
 	// Random anime pool for DDoS-proof truly random "Surprise Me"
 	randomPool      []Anime
@@ -38,7 +40,7 @@ type Client struct {
 
 const jikanSlowLogThreshold = 750 * time.Millisecond
 
-func NewClient(queries *db.Queries) *Client {
+func NewClient(queries *db.Queries, metrics *observability.Metrics) *Client {
 	return &Client{
 		httpClient: &http.Client{
 			Timeout: 10 * time.Second,
@@ -51,6 +53,7 @@ func NewClient(queries *db.Queries) *Client {
 		},
 		baseURL:     "https://api.jikan.moe/v4",
 		db:          queries,
+		metrics:     metrics,
 		retrySignal: make(chan struct{}, 1),
 		refreshSem:  make(chan struct{}, 4),
 		randomPool:  make([]Anime, 0),
@@ -262,11 +265,18 @@ func (c *Client) getCache(parentCtx context.Context, key string, out any) bool {
 
 	data, err := c.db.GetJikanCache(ctx, key)
 	if err != nil {
+		c.metrics.ObserveCache("jikan", "miss")
 		return false
 	}
 
 	err = json.Unmarshal([]byte(data), out)
-	return err == nil
+	if err != nil {
+		c.metrics.ObserveCache("jikan", "miss")
+		return false
+	}
+
+	c.metrics.ObserveCache("jikan", "hit")
+	return true
 }
 
 // getStaleCache retrieves expired-but-available cache by key.
@@ -276,11 +286,18 @@ func (c *Client) getStaleCache(parentCtx context.Context, key string, out any) b
 
 	data, err := c.db.GetJikanCacheStale(ctx, key)
 	if err != nil {
+		c.metrics.ObserveCache("jikan_stale", "miss")
 		return false
 	}
 
 	err = json.Unmarshal([]byte(data), out)
-	return err == nil
+	if err != nil {
+		c.metrics.ObserveCache("jikan_stale", "miss")
+		return false
+	}
+
+	c.metrics.ObserveCache("jikan_stale", "hit")
+	return true
 }
 
 // setCache stores data in cache with specified TTL.
@@ -425,7 +442,9 @@ func (c *Client) fetchWithRetry(ctx context.Context, urlStr string, out any) err
 	maxRetries := 5
 	startedAt := time.Now()
 	attempts := 0
+	endpoint := metricsEndpoint(urlStr)
 	logAndReturn := func(statusCode int, err error) error {
+		c.metrics.ObserveJikanRequest(endpoint, statusCode, time.Since(startedAt), err)
 		logJikanUpstream(urlStr, statusCode, attempts, startedAt, err)
 		return err
 	}
@@ -506,3 +525,38 @@ func (c *Client) fetchWithRetry(ctx context.Context, urlStr string, out any) err
 
 	return logAndReturn(0, fmt.Errorf("max retries exceeded for %s", urlStr))
 }
+
+func metricsEndpoint(urlStr string) string {
+	trimmed := strings.TrimSpace(urlStr)
+	if trimmed == "" {
+		return "unknown"
+	}
+
+	prefix := "https://api.jikan.moe/v4"
+	if strings.HasPrefix(trimmed, prefix) {
+		trimmed = strings.TrimPrefix(trimmed, prefix)
+	}
+
+	if idx := strings.Index(trimmed, "?"); idx >= 0 {
+		trimmed = trimmed[:idx]
+	}
+
+	parts := strings.Split(trimmed, "/")
+	out := make([]string, 0, len(parts))
+	for _, part := range parts {
+		if part == "" {
+			continue
+		}
+		if _, err := strconv.Atoi(part); err == nil {
+			out = append(out, "{id}")
+			continue
+		}
+		out = append(out, part)
+	}
+
+	if len(out) == 0 {
+		return "/"
+	}
+
+	return "/" + strings.Join(out, "/")
+}

integrations/jikan/client_test.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"io"
 	"mal/internal/db"
+	"mal/internal/observability"
 	"net/http"
 	"strings"
 	"testing"
@@ -41,7 +42,7 @@ func TestGetWithCacheReturnsStaleAndRefreshesAsync(t *testing.T) {
 	}
 
 	queries := db.New(sqlDB)
-	client := NewClient(queries)
+	client := NewClient(queries, observability.NewMetrics())
 	stale := TopAnimeResponse{Data: []Anime{{MalID: 1, Title: "stale"}}}
 	staleBytes, err := json.Marshal(stale)
 	if err != nil {

integrations/jikan/module.go

@@ -1,8 +1,6 @@
 package jikan
 
-import (
+import "go.uber.org/fx"
-	"go.uber.org/fx"
-)
 
 var Module = fx.Options(
 	fx.Provide(NewClient),

internal/auth/middleware/middleware.go

@@ -16,6 +16,7 @@ func AuthMiddleware(svc domain.AuthService) gin.HandlerFunc {
 		if path == "/login" || path == "/logout" ||
 			strings.HasPrefix(path, "/static") ||
 			strings.HasPrefix(path, "/dist") ||
+			path == "/metrics" ||
 			path == "/api/auth/login" {
 			c.Next()
 			return

internal/episodes/module.go

@@ -9,6 +9,7 @@ import (
 	"mal/internal/db"
 	"mal/internal/domain"
 	episodeService "mal/internal/episodes/service"
+	"mal/internal/observability"
 
 	"go.uber.org/fx"
 )
@@ -22,10 +23,10 @@ var Module = fx.Options(
 	fx.Provide(
 		episodeAvailabilityEnabled,
 		fx.Annotate(
-			func(queries *db.Queries, jikanClient *jikan.Client, providers []domain.EpisodeAvailabilityProvider, enabled bool) domain.EpisodeService {
+			func(queries *db.Queries, jikanClient *jikan.Client, providers []domain.EpisodeAvailabilityProvider, enabled bool, metrics *observability.Metrics) domain.EpisodeService {
-				return episodeService.NewEpisodeService(queries, jikanClient, providers, enabled)
+				return episodeService.NewEpisodeService(queries, jikanClient, providers, enabled, metrics)
 			},
-			fx.ParamTags(``, ``, ``, ``),
+			fx.ParamTags(``, ``, ``, ``, ``),
 		),
 	),
 	fx.Provide(func(p *allanime.AllAnimeProvider) []domain.EpisodeAvailabilityProvider {

internal/episodes/service/service.go

@@ -10,6 +10,7 @@ import (
 	"mal/integrations/jikan"
 	"mal/internal/db"
 	"mal/internal/domain"
+	"mal/internal/observability"
 	"sort"
 	"strings"
 	"time"
@@ -34,19 +35,21 @@ type EpisodeService struct {
 	providers []domain.EpisodeAvailabilityProvider
 	clock     Clock
 	enabled   bool
+	metrics   *observability.Metrics
 }
 
-func NewEpisodeService(queries *db.Queries, jikanClient *jikan.Client, providers []domain.EpisodeAvailabilityProvider, enabled bool) domain.EpisodeService {
+func NewEpisodeService(queries *db.Queries, jikanClient *jikan.Client, providers []domain.EpisodeAvailabilityProvider, enabled bool, metrics *observability.Metrics) domain.EpisodeService {
-	return NewEpisodeServiceWithClock(queries, jikanClient, providers, enabled, realClock{})
+	return NewEpisodeServiceWithClock(queries, jikanClient, providers, enabled, realClock{}, metrics)
 }
 
-func NewEpisodeServiceWithClock(queries *db.Queries, jikanClient *jikan.Client, providers []domain.EpisodeAvailabilityProvider, enabled bool, clock Clock) *EpisodeService {
+func NewEpisodeServiceWithClock(queries *db.Queries, jikanClient *jikan.Client, providers []domain.EpisodeAvailabilityProvider, enabled bool, clock Clock, metrics *observability.Metrics) *EpisodeService {
 	return &EpisodeService{
 		queries:   queries,
 		jikan:     jikanClient,
 		providers: providers,
 		clock:     clock,
 		enabled:   enabled,
+		metrics:   metrics,
 	}
 }
 
@@ -143,14 +146,20 @@ func (s *EpisodeService) providerID(ctx context.Context, anime domain.Anime, pro
 	})
 	if err == nil {
 		if row.FailedUntil.Valid && row.FailedUntil.Time.After(s.clock.Now()) {
+			s.metrics.ObserveCache("episode_provider_mapping", "hit")
 			return "", fmt.Errorf("cached provider mapping failure active until %s: %s", row.FailedUntil.Time.Format(time.RFC3339), row.LastError)
 		}
 		if strings.TrimSpace(row.ProviderShowID) != "" {
+			s.metrics.ObserveCache("episode_provider_mapping", "hit")
 			log.Printf("episodes: provider id cache hit anime_id=%d provider=%s provider_id=%s", anime.MalID, provider.Name(), row.ProviderShowID)
 			return row.ProviderShowID, nil
 		}
+		s.metrics.ObserveCache("episode_provider_mapping", "miss")
 	} else if !errors.Is(err, sql.ErrNoRows) {
+		s.metrics.ObserveCache("episode_provider_mapping", "miss")
 		log.Printf("episodes: provider id cache read failed anime_id=%d provider=%s error=%v", anime.MalID, provider.Name(), err)
+	} else {
+		s.metrics.ObserveCache("episode_provider_mapping", "miss")
 	}
 
 	providerID, err := provider.ResolveEpisodeProviderID(ctx, anime.MalID, titles)
@@ -256,31 +265,38 @@ func (s *EpisodeService) markFailure(ctx context.Context, anime domain.Anime, ca
 func (s *EpisodeService) getCached(ctx context.Context, animeID int) (domain.CanonicalEpisodeList, bool) {
 	row, err := s.queries.GetEpisodeAvailabilityCache(ctx, int64(animeID))
 	if err != nil {
+		s.metrics.ObserveCache("episode_availability", "miss")
 		return domain.CanonicalEpisodeList{}, false
 	}
 	var payload domain.CanonicalEpisodeList
 	if err := json.Unmarshal([]byte(row.Data), &payload); err != nil {
+		s.metrics.ObserveCache("episode_availability", "miss")
 		log.Printf("episodes: invalid cached payload anime_id=%d error=%v", animeID, err)
 		return domain.CanonicalEpisodeList{}, false
 	}
+	s.metrics.ObserveCache("episode_availability", "hit")
 	return payload, true
 }
 
 func (s *EpisodeService) getFreshCached(ctx context.Context, animeID int) (domain.CanonicalEpisodeList, bool) {
 	row, err := s.queries.GetEpisodeAvailabilityCache(ctx, int64(animeID))
 	if err != nil {
+		s.metrics.ObserveCache("episode_availability_fresh", "miss")
 		return domain.CanonicalEpisodeList{}, false
 	}
 	if row.NextRefreshAt.Valid && !row.NextRefreshAt.Time.After(s.clock.Now()) {
+		s.metrics.ObserveCache("episode_availability_fresh", "miss")
 		log.Printf("episodes: cached availability due for refresh anime_id=%d next_refresh=%s", animeID, row.NextRefreshAt.Time.Format(time.RFC3339))
 		return domain.CanonicalEpisodeList{}, false
 	}
 
 	var payload domain.CanonicalEpisodeList
 	if err := json.Unmarshal([]byte(row.Data), &payload); err != nil {
+		s.metrics.ObserveCache("episode_availability_fresh", "miss")
 		log.Printf("episodes: invalid cached payload anime_id=%d error=%v", animeID, err)
 		return domain.CanonicalEpisodeList{}, false
 	}
+	s.metrics.ObserveCache("episode_availability_fresh", "hit")
 	log.Printf("episodes: served cached availability anime_id=%d episodes=%d next_refresh=%s", animeID, len(payload.Episodes), payload.NextRefreshAt)
 	return payload, true
 }

internal/episodes/worker.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"log"
 	"mal/internal/domain"
+	"mal/internal/observability"
 	"time"
 
 	"go.uber.org/fx"
@@ -11,7 +12,7 @@ import (
 
 const workerInterval = time.Minute
 
-func RegisterWorker(lc fx.Lifecycle, svc domain.EpisodeService) {
+func RegisterWorker(lc fx.Lifecycle, svc domain.EpisodeService, metrics *observability.Metrics) {
 	ctx, cancel := context.WithCancel(context.Background())
 
 	lc.Append(fx.Hook{
@@ -23,7 +24,10 @@ func RegisterWorker(lc fx.Lifecycle, svc domain.EpisodeService) {
 
 				for {
 					if err := svc.RefreshTrackedDue(ctx, 25); err != nil {
+						metrics.ObserveWorkerTick("episodes_availability", err)
 						log.Printf("episodes: availability worker tick failed error=%v", err)
+					} else {
+						metrics.ObserveWorkerTick("episodes_availability", nil)
 					}
 
 					select {

internal/observability/metrics.go

@@ -0,0 +1,292 @@
+package observability
+
+import (
+	"fmt"
+	"net/http"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+)
+
+var defaultDurationBuckets = []float64{
+	0.005,
+	0.01,
+	0.025,
+	0.05,
+	0.1,
+	0.25,
+	0.5,
+	1,
+	2.5,
+	5,
+	10,
+}
+
+type counterSample struct {
+	labels map[string]string
+	value  uint64
+}
+
+type histogramSample struct {
+	labels  map[string]string
+	buckets []uint64
+	count   uint64
+	sum     float64
+}
+
+type counterVec struct {
+	mu         sync.Mutex
+	labelNames []string
+	samples    map[string]*counterSample
+}
+
+type histogramVec struct {
+	mu         sync.Mutex
+	labelNames []string
+	bounds     []float64
+	samples    map[string]*histogramSample
+}
+
+type Metrics struct {
+	httpRequests       *counterVec
+	httpRequestLatency *histogramVec
+	jikanRequests      *counterVec
+	jikanRequestErrors *counterVec
+	jikanLatency       *histogramVec
+	workerTicks        *counterVec
+	cacheOperations    *counterVec
+}
+
+func NewMetrics() *Metrics {
+	return &Metrics{
+		httpRequests:       newCounterVec("method", "route", "status"),
+		httpRequestLatency: newHistogramVec(defaultDurationBuckets, "method", "route", "status"),
+		jikanRequests:      newCounterVec("endpoint", "status"),
+		jikanRequestErrors: newCounterVec("endpoint", "status"),
+		jikanLatency:       newHistogramVec(defaultDurationBuckets, "endpoint", "status"),
+		workerTicks:        newCounterVec("worker", "result"),
+		cacheOperations:    newCounterVec("cache", "result"),
+	}
+}
+
+func (m *Metrics) Handler() http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/plain; version=0.0.4; charset=utf-8")
+		w.WriteHeader(http.StatusOK)
+		m.writePrometheus(w)
+	})
+}
+
+func (m *Metrics) ObserveHTTPRequest(method string, route string, status int, duration time.Duration) {
+	statusLabel := strconv.Itoa(status)
+	m.httpRequests.Inc(method, route, statusLabel)
+	m.httpRequestLatency.Observe(duration.Seconds(), method, route, statusLabel)
+}
+
+func (m *Metrics) ObserveJikanRequest(endpoint string, status int, duration time.Duration, err error) {
+	statusLabel := strconv.Itoa(status)
+	m.jikanRequests.Inc(endpoint, statusLabel)
+	m.jikanLatency.Observe(duration.Seconds(), endpoint, statusLabel)
+	if err != nil || status >= http.StatusBadRequest {
+		m.jikanRequestErrors.Inc(endpoint, statusLabel)
+	}
+}
+
+func (m *Metrics) ObserveWorkerTick(worker string, err error) {
+	if err != nil {
+		m.workerTicks.Inc(worker, "failure")
+		return
+	}
+	m.workerTicks.Inc(worker, "success")
+}
+
+func (m *Metrics) ObserveCache(cache string, result string) {
+	m.cacheOperations.Inc(cache, result)
+}
+
+func (m *Metrics) writePrometheus(w http.ResponseWriter) {
+	writeCounterMetric(w, "mal_http_requests_total", "Total HTTP requests by method, route, and status.", m.httpRequests.snapshot())
+	writeHistogramMetric(w, "mal_http_request_duration_seconds", "HTTP request latency in seconds.", m.httpRequestLatency.snapshot(), m.httpRequestLatency.bounds)
+	writeCounterMetric(w, "mal_jikan_upstream_requests_total", "Total upstream Jikan requests by endpoint and status.", m.jikanRequests.snapshot())
+	writeCounterMetric(w, "mal_jikan_upstream_errors_total", "Total upstream Jikan errors by endpoint and status.", m.jikanRequestErrors.snapshot())
+	writeHistogramMetric(w, "mal_jikan_upstream_request_duration_seconds", "Upstream Jikan request latency in seconds.", m.jikanLatency.snapshot(), m.jikanLatency.bounds)
+	writeCounterMetric(w, "mal_worker_ticks_total", "Total background worker ticks by worker and result.", m.workerTicks.snapshot())
+	writeCounterMetric(w, "mal_cache_operations_total", "Total cache hits and misses by cache name.", m.cacheOperations.snapshot())
+}
+
+func newCounterVec(labelNames ...string) *counterVec {
+	return &counterVec{
+		labelNames: append([]string(nil), labelNames...),
+		samples:    make(map[string]*counterSample),
+	}
+}
+
+func (c *counterVec) Inc(labelValues ...string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	key, labels := buildLabelKey(c.labelNames, labelValues)
+	sample, ok := c.samples[key]
+	if !ok {
+		sample = &counterSample{labels: labels}
+		c.samples[key] = sample
+	}
+	sample.value++
+}
+
+func (c *counterVec) snapshot() []counterSample {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	keys := sortedCounterSampleKeys(c.samples)
+	out := make([]counterSample, 0, len(keys))
+	for _, key := range keys {
+		sample := c.samples[key]
+		out = append(out, counterSample{
+			labels: copyLabels(sample.labels),
+			value:  sample.value,
+		})
+	}
+	return out
+}
+
+func newHistogramVec(bounds []float64, labelNames ...string) *histogramVec {
+	return &histogramVec{
+		labelNames: append([]string(nil), labelNames...),
+		bounds:     append([]float64(nil), bounds...),
+		samples:    make(map[string]*histogramSample),
+	}
+}
+
+func (h *histogramVec) Observe(value float64, labelValues ...string) {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	key, labels := buildLabelKey(h.labelNames, labelValues)
+	sample, ok := h.samples[key]
+	if !ok {
+		sample = &histogramSample{
+			labels:  labels,
+			buckets: make([]uint64, len(h.bounds)),
+		}
+		h.samples[key] = sample
+	}
+
+	sample.count++
+	sample.sum += value
+	for idx, bound := range h.bounds {
+		if value <= bound {
+			sample.buckets[idx]++
+		}
+	}
+}
+
+func (h *histogramVec) snapshot() []histogramSample {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	keys := sortedHistogramSampleKeys(h.samples)
+	out := make([]histogramSample, 0, len(keys))
+	for _, key := range keys {
+		sample := h.samples[key]
+		buckets := make([]uint64, len(sample.buckets))
+		copy(buckets, sample.buckets)
+		out = append(out, histogramSample{
+			labels:  copyLabels(sample.labels),
+			buckets: buckets,
+			count:   sample.count,
+			sum:     sample.sum,
+		})
+	}
+	return out
+}
+
+func buildLabelKey(labelNames []string, labelValues []string) (string, map[string]string) {
+	if len(labelNames) != len(labelValues) {
+		panic("label cardinality mismatch")
+	}
+
+	labels := make(map[string]string, len(labelNames))
+	parts := make([]string, 0, len(labelNames)*2)
+	for idx, name := range labelNames {
+		value := labelValues[idx]
+		labels[name] = value
+		parts = append(parts, name, value)
+	}
+	return strings.Join(parts, "\xff"), labels
+}
+
+func copyLabels(labels map[string]string) map[string]string {
+	out := make(map[string]string, len(labels))
+	for key, value := range labels {
+		out[key] = value
+	}
+	return out
+}
+
+func sortedCounterSampleKeys(samples map[string]*counterSample) []string {
+	keys := make([]string, 0, len(samples))
+	for key := range samples {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+	return keys
+}
+
+func sortedHistogramSampleKeys(samples map[string]*histogramSample) []string {
+	keys := make([]string, 0, len(samples))
+	for key := range samples {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+	return keys
+}
+
+func writeCounterMetric(w http.ResponseWriter, name string, help string, samples []counterSample) {
+	_, _ = fmt.Fprintf(w, "# HELP %s %s\n", name, help)
+	_, _ = fmt.Fprintf(w, "# TYPE %s counter\n", name)
+	for _, sample := range samples {
+		_, _ = fmt.Fprintf(w, "%s%s %d\n", name, formatLabels(sample.labels), sample.value)
+	}
+}
+
+func writeHistogramMetric(w http.ResponseWriter, name string, help string, samples []histogramSample, bounds []float64) {
+	_, _ = fmt.Fprintf(w, "# HELP %s %s\n", name, help)
+	_, _ = fmt.Fprintf(w, "# TYPE %s histogram\n", name)
+	for _, sample := range samples {
+		for idx, bound := range bounds {
+			labels := copyLabels(sample.labels)
+			labels["le"] = formatFloat(bound)
+			_, _ = fmt.Fprintf(w, "%s_bucket%s %d\n", name, formatLabels(labels), sample.buckets[idx])
+		}
+		labels := copyLabels(sample.labels)
+		labels["le"] = "+Inf"
+		_, _ = fmt.Fprintf(w, "%s_bucket%s %d\n", name, formatLabels(labels), sample.count)
+		_, _ = fmt.Fprintf(w, "%s_sum%s %s\n", name, formatLabels(sample.labels), formatFloat(sample.sum))
+		_, _ = fmt.Fprintf(w, "%s_count%s %d\n", name, formatLabels(sample.labels), sample.count)
+	}
+}
+
+func formatLabels(labels map[string]string) string {
+	if len(labels) == 0 {
+		return ""
+	}
+
+	keys := make([]string, 0, len(labels))
+	for key := range labels {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+
+	parts := make([]string, 0, len(keys))
+	for _, key := range keys {
+		parts = append(parts, fmt.Sprintf(`%s=%q`, key, labels[key]))
+	}
+	return "{" + strings.Join(parts, ",") + "}"
+}
+
+func formatFloat(value float64) string {
+	return strconv.FormatFloat(value, 'f', -1, 64)
+}

internal/observability/metrics_test.go

@@ -0,0 +1,47 @@
+package observability
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestMetricsHandlerRendersPrometheusFamilies(t *testing.T) {
+	metrics := NewMetrics()
+	metrics.ObserveHTTPRequest(http.MethodGet, "/anime/:id", http.StatusOK, 125*time.Millisecond)
+	metrics.ObserveJikanRequest("/anime/{id}", http.StatusTooManyRequests, 800*time.Millisecond, assertErr{})
+	metrics.ObserveWorkerTick("episodes_availability", nil)
+	metrics.ObserveCache("jikan", "hit")
+	metrics.ObserveCache("episode_availability", "miss")
+
+	req := httptest.NewRequest(http.MethodGet, "/metrics", nil)
+	rec := httptest.NewRecorder()
+	metrics.Handler().ServeHTTP(rec, req)
+
+	body, err := io.ReadAll(rec.Result().Body)
+	if err != nil {
+		t.Fatalf("read body: %v", err)
+	}
+
+	text := string(body)
+	assertContains(t, text, `mal_http_requests_total{method="GET",route="/anime/:id",status="200"} 1`)
+	assertContains(t, text, `mal_http_request_duration_seconds_count{method="GET",route="/anime/:id",status="200"} 1`)
+	assertContains(t, text, `mal_jikan_upstream_requests_total{endpoint="/anime/{id}",status="429"} 1`)
+	assertContains(t, text, `mal_jikan_upstream_errors_total{endpoint="/anime/{id}",status="429"} 1`)
+	assertContains(t, text, `mal_worker_ticks_total{result="success",worker="episodes_availability"} 1`)
+	assertContains(t, text, `mal_cache_operations_total{cache="episode_availability",result="miss"} 1`)
+}
+
+type assertErr struct{}
+
+func (assertErr) Error() string { return "boom" }
+
+func assertContains(t *testing.T, text string, want string) {
+	t.Helper()
+	if !strings.Contains(text, want) {
+		t.Fatalf("missing metric line %q in:\n%s", want, text)
+	}
+}

internal/server/observability.go

@@ -2,13 +2,14 @@ package server
 
 import (
 	"log"
+	"mal/internal/observability"
 	"strconv"
 	"time"
 
 	"github.com/gin-gonic/gin"
 )
 
-func RequestLogger() gin.HandlerFunc {
+func RequestLogger(metrics *observability.Metrics) gin.HandlerFunc {
 	return func(c *gin.Context) {
 		start := time.Now()
 		path := c.Request.URL.Path
@@ -21,6 +22,9 @@ func RequestLogger() gin.HandlerFunc {
 			route = path
 		}
 
+		duration := time.Since(start)
+		metrics.ObserveHTTPRequest(c.Request.Method, route, c.Writer.Status(), duration)
+
 		log.Printf(
 			"http_request method=%s route=%s path=%s query=%s status=%d duration_ms=%.2f bytes=%d client_ip=%s errors=%s",
 			c.Request.Method,
@@ -28,7 +32,7 @@ func RequestLogger() gin.HandlerFunc {
 			strconv.Quote(path),
 			strconv.Quote(query),
 			c.Writer.Status(),
-			float64(time.Since(start).Microseconds())/1000,
+			float64(duration.Microseconds())/1000,
 			c.Writer.Size(),
 			strconv.Quote(c.ClientIP()),
 			strconv.Quote(c.Errors.ByType(gin.ErrorTypePrivate).String()),

internal/server/server.go

@@ -3,6 +3,7 @@ package server
 import (
 	"context"
 	"log"
+	"mal/internal/observability"
 	"net/http"
 	"os"
 	"time"
@@ -13,18 +14,20 @@ import (
 )
 
 var Module = fx.Options(
+	fx.Provide(observability.NewMetrics),
 	fx.Provide(ProvideRouter),
 	fx.Invoke(RunServer),
 )
 
-func ProvideRouter(htmlRender render.HTMLRender) *gin.Engine {
+func ProvideRouter(htmlRender render.HTMLRender, metrics *observability.Metrics) *gin.Engine {
 	if os.Getenv("GIN_MODE") == "" {
 		gin.SetMode(gin.ReleaseMode)
 	}
 	r := gin.New()
-	r.Use(CORSMiddleware(), RequestLogger(), gin.Recovery())
+	r.Use(CORSMiddleware(), RequestLogger(metrics), gin.Recovery())
 	r.Static("/static", "./static")
 	r.Static("/dist", "./dist")
+	r.GET("/metrics", gin.WrapH(metrics.Handler()))
 	r.HTMLRender = htmlRender
 	return r
 }

internal/server/server_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"io"
 	"log"
+	"mal/internal/observability"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -42,7 +43,7 @@ func TestRequestLoggerUsesMatchedRoute(t *testing.T) {
 	defer log.SetOutput(previousOutput)
 
 	router := gin.New()
-	router.Use(RequestLogger())
+	router.Use(RequestLogger(observability.NewMetrics()))
 	router.GET("/anime/:id", func(c *gin.Context) {
 		c.String(http.StatusOK, "ok")
 	})