feat(metrics): Prometheus collectors + typed API + HTTP middleware (ADR-030, #15)

New internal/metrics package: AI metrics (summarize/caption/chat latency, LLM
tokens), session metrics (requests+latency by bounded route pattern, logins), and
a /metrics Handler. Adapters call a typed API; never touch prometheus types.
Dep: github.com/prometheus/client_golang (standard Go client; cluster runs
prometheus-operator). TDD: collectors + middleware route-pattern cardinality
covered.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/metrics/metrics_test.go

@@ -0,0 +1,92 @@
+package metrics
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/testutil"
+	dto "github.com/prometheus/client_model/go"
+	"github.com/stretchr/testify/require"
+)
+
+// histCount reads a histogram child's observation count (testutil.ToFloat64 only
+// works on counters/gauges; a histogram's WithLabelValues child is an Observer).
+func histCount(t *testing.T, o prometheus.Observer) uint64 {
+	t.Helper()
+	m, ok := o.(prometheus.Metric)
+	require.True(t, ok, "histogram child must be a prometheus.Metric")
+	var d dto.Metric
+	require.NoError(t, m.Write(&d))
+	return d.GetHistogram().GetSampleCount()
+}
+
+// TestObserveSummarizeRecordsModelOutcomeFallback: a success observation lands on
+// the right model/outcome/fallback series.
+func TestObserveSummarizeRecordsModelOutcomeFallback(t *testing.T) {
+	before := histCount(t, summarizeDuration.WithLabelValues("koala/phi4-mini", "success", "false"))
+	ObserveSummarize("koala/phi4-mini", "success", false, 1200*time.Millisecond)
+	after := histCount(t, summarizeDuration.WithLabelValues("koala/phi4-mini", "success", "false"))
+	require.Equal(t, before+1, after, "one success observation recorded for the model")
+}
+
+// TestObserveSummarizeRecordsFailureOutcomes: error and parse_error are distinct
+// series so a fallback chain's failures are visible.
+func TestObserveSummarizeRecordsFailureOutcomes(t *testing.T) {
+	e0 := histCount(t, summarizeDuration.WithLabelValues("m", "error", "false"))
+	p0 := histCount(t, summarizeDuration.WithLabelValues("m", "parse_error", "false"))
+	ObserveSummarize("m", "error", false, time.Second)
+	ObserveSummarize("m", "parse_error", false, time.Second)
+	require.Equal(t, e0+1, histCount(t, summarizeDuration.WithLabelValues("m", "error", "false")))
+	require.Equal(t, p0+1, histCount(t, summarizeDuration.WithLabelValues("m", "parse_error", "false")))
+}
+
+func TestObserveCaptionFetchByOutcome(t *testing.T) {
+	b := histCount(t, captionFetchDuration.WithLabelValues("captions"))
+	ObserveCaptionFetch("captions", 3*time.Second)
+	require.Equal(t, b+1, histCount(t, captionFetchDuration.WithLabelValues("captions")))
+}
+
+func TestChatAnswerLatencyRecorded(t *testing.T) {
+	b := histCount(t, chatDuration.WithLabelValues("iguana/gemma4-26b"))
+	ObserveChat("iguana/gemma4-26b", 2*time.Second)
+	require.Equal(t, b+1, histCount(t, chatDuration.WithLabelValues("iguana/gemma4-26b")))
+}
+
+// TestRecordTokens: prompt + completion land on their kind series; zero is skipped.
+func TestRecordTokens(t *testing.T) {
+	p0 := testutil.ToFloat64(llmTokens.WithLabelValues("m", "prompt"))
+	c0 := testutil.ToFloat64(llmTokens.WithLabelValues("m", "completion"))
+	RecordTokens("m", 100, 40)
+	RecordTokens("m", 0, 0) // skipped, no panic
+	require.Equal(t, p0+100, testutil.ToFloat64(llmTokens.WithLabelValues("m", "prompt")))
+	require.Equal(t, c0+40, testutil.ToFloat64(llmTokens.WithLabelValues("m", "completion")))
+}
+
+func TestLoginCounted(t *testing.T) {
+	b := testutil.ToFloat64(logins)
+	IncLogin()
+	require.Equal(t, b+1, testutil.ToFloat64(logins))
+}
+
+// TestHTTPMiddlewareRecordsByRoutePattern: the request is counted under the bounded
+// registered pattern (r.Pattern after routing), not the raw path with its ids.
+func TestHTTPMiddlewareRecordsByRoutePattern(t *testing.T) {
+	mux := http.NewServeMux()
+	mux.HandleFunc("GET /v/{videoId}", func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusTeapot)
+	})
+	h := HTTPMiddleware(mux)
+
+	before := testutil.ToFloat64(httpRequests.WithLabelValues("GET", "GET /v/{videoId}", "418"))
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/v/abc-123", nil))
+
+	require.Equal(t, http.StatusTeapot, rec.Code)
+	after := testutil.ToFloat64(httpRequests.WithLabelValues("GET", "GET /v/{videoId}", "418"))
+	require.Equal(t, before+1, after, "counted under the pattern, not /v/abc-123")
+	require.Equal(t, float64(0), testutil.ToFloat64(httpRequests.WithLabelValues("GET", "/v/abc-123", "418")),
+		"raw path must never be a label value")
+}