feat: add observability metrics

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)
+	}
+}