feat: Prometheus metrics endpoint for knox nodes

Refs #2

- /metrics served on a dedicated port (KNOX_METRICS_ADDR, default
  localhost:8932) via prometheus/client_golang, with Go runtime +
  process collectors
- DB-derived gauges refreshed per scrape: observations by source,
  last-24h observations, entries, projects, sessions, pending
  reflections, threads by status, peers, observations by origin node,
  knowledge vector (max hcl per node)
- live gossip counters (pulls/pushes, observations pulled/pushed,
  errors) incremented during the anti-entropy sweep; Run accepts an
  optional metrics handle (nil for one-shot CLI)
- knox_node_info{node_id,name} for scrape identification
- internal/metrics package + db MetricsSnapshot; tests for snapshot,
  scrape output, and counter increments

internal/metrics/metrics_test.go

@@ -0,0 +1,114 @@
+package metrics
+
+import (
+	"io"
+	"net/http/httptest"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/david/knox/internal/db"
+	"github.com/prometheus/client_golang/prometheus/testutil"
+)
+
+func tmpKdb(t *testing.T) *db.KnoxDB {
+	t.Helper()
+	k, err := db.Open(filepath.Join(t.TempDir(), "index.db"))
+	if err != nil {
+		t.Fatalf("open db: %v", err)
+	}
+	t.Cleanup(func() { k.Close() })
+	return k
+}
+
+func seed(t *testing.T, k *db.KnoxDB, src string, n int) {
+	t.Helper()
+	for i := 0; i < n; i++ {
+		_, _, err := k.RecordObservation(db.ObservationRecord{
+			Fingerprint:     "fp-" + src + "-" + string(rune('a'+i)),
+			SourceID:        src,
+			SourcePath:      src,
+			Project:         "test",
+			ContentType:     "test",
+			Title:           src,
+			Summary:         "s",
+			CreatedAt:       "2026-08-29T00:00:00Z",
+			Confidence:      0.9,
+			IngesterVersion: "itest/v1",
+		})
+		if err != nil {
+			t.Fatalf("seed: %v", err)
+		}
+	}
+}
+
+func TestMetricsSnapshot(t *testing.T) {
+	k := tmpKdb(t)
+	seed(t, k, "git", 2)
+	seed(t, k, "browser-history", 3)
+
+	s, err := k.MetricsSnapshot()
+	if err != nil {
+		t.Fatalf("snapshot: %v", err)
+	}
+	if s.Observations != 5 {
+		t.Errorf("observations = %d, want 5", s.Observations)
+	}
+	if s.BySource["git"] != 2 || s.BySource["browser-history"] != 3 {
+		t.Errorf("by source = %v", s.BySource)
+	}
+	if s.KnowledgeVector[k.NodeID()] == 0 {
+		t.Errorf("knowledge vector missing own node")
+	}
+	if s.ByOriginNode[k.NodeID()] != 5 {
+		t.Errorf("by origin node = %v", s.ByOriginNode)
+	}
+}
+
+func TestMetricsScrape(t *testing.T) {
+	k := tmpKdb(t)
+	seed(t, k, "git", 2)
+
+	m := New(k, "testnode")
+	h := m.Handler()
+
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, httptest.NewRequest("GET", "/metrics", nil))
+
+	body, _ := io.ReadAll(rec.Body)
+	out := string(body)
+	for _, want := range []string{
+		`knox_node_info{name="testnode"`,
+		`knox_observations_total{source_id="git"} 2`,
+		`knox_gossip_pulls_total 0`,
+		"go_goroutines",
+		"process_cpu_seconds_total",
+	} {
+		if !strings.Contains(out, want) {
+			t.Errorf("scrape output missing %q", want)
+		}
+	}
+}
+
+func TestMetricsCounters(t *testing.T) {
+	k := tmpKdb(t)
+	m := New(k, "t")
+
+	// Manually drive counters through the Metrics API.
+	m.IncrementPull(3)
+	m.IncrementPush(7)
+	m.IncrementErrors()
+
+	if got := testutil.ToFloat64(m.pullsTotal); got != 1 {
+		t.Errorf("pullsTotal = %v, want 1", got)
+	}
+	if got := testutil.ToFloat64(m.obsPulledTotal); got != 3 {
+		t.Errorf("obsPulled = %v, want 3", got)
+	}
+	if got := testutil.ToFloat64(m.obsPushedTotal); got != 7 {
+		t.Errorf("obsPushed = %v, want 7", got)
+	}
+	if got := testutil.ToFloat64(m.errorsTotal); got != 1 {
+		t.Errorf("errorsTotal = %v, want 1", got)
+	}
+}