Stage 12: observability & performance (OTel/OTLP, domain metrics, guest GC)

- pkg/telemetry: shared OTel provider bootstrap (none/stdout/otlp + W3C
  propagators + Go runtime metrics); backend/internal/telemetry becomes a thin
  facade keeping its gin middleware.
- Telemetry parity: gateway and the Telegram connector gain telemetry runtimes
  and config (GATEWAY_/TELEGRAM_ SERVICE_NAME + OTEL_*); otelgrpc instruments the
  backend push server, the gateway's backend+connector clients and the connector
  server. Default exporter stays none (collector/dashboards are Stage 14).
- Operational metrics (variant attribute on game-scoped ones): game_replay_duration,
  game_move_validate_duration, games_started_total, games_abandoned_total,
  game_cache_active, chat_messages_total{kind}, gateway edge_request_duration.
  Wired via the SetMetrics setter pattern (default no-op meter).
- TODO-3: account.GuestReaper deletes guests with no game seat past
  BACKEND_GUEST_RETENTION (default 30d, swept every BACKEND_GUEST_REAP_INTERVAL).
- Tests: pkg/telemetry exporter selection; game/social/edge metric recording via
  a manual reader; config (otlp accepted, guest knobs); inttest guest reaper.
- Docs: PLAN.md re-scopes Stage 12 and adds Stage 13 (alphabet-on-wire) + Stage 14
  (CI/deploy) with the agreed dictionary-versioning resolution; ARCHITECTURE 11/13,
  TESTING, the three READMEs and FUNCTIONAL(+ru) updated.

backend/internal/social/chat.go

@@ -77,6 +77,7 @@ func (svc *Service) PostMessage(ctx context.Context, gameID, senderID uuid.UUID,
 	if err != nil {
 		return Message{}, err
 	}
+	svc.metrics.recordChat(ctx, kindMessage)
 	svc.emitChat(seats, senderID, msg)
 	return msg, nil
 }
@@ -110,6 +111,7 @@ func (svc *Service) Nudge(ctx context.Context, gameID, senderID uuid.UUID) (Mess
 	if err != nil {
 		return Message{}, err
 	}
+	svc.metrics.recordChat(ctx, kindNudge)
 	if toMove >= 0 && toMove < len(seats) {
 		svc.pub.Publish(notify.Nudge(seats[toMove], gameID, senderID))
 	}

backend/internal/social/metrics.go

@@ -0,0 +1,49 @@
+package social
+
+import (
+	"context"
+
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/metric"
+	"go.opentelemetry.io/otel/metric/noop"
+)
+
+// meterName scopes the social domain's OpenTelemetry instruments.
+const meterName = "scrabble/backend/social"
+
+// socialMetrics holds the social domain's operational instruments. It defaults to
+// no-ops (see defaultSocialMetrics); SetMetrics installs the real meter during
+// startup wiring.
+type socialMetrics struct {
+	messages metric.Int64Counter
+}
+
+// defaultSocialMetrics returns instruments backed by a no-op meter.
+func defaultSocialMetrics() *socialMetrics {
+	return newSocialMetrics(noop.NewMeterProvider().Meter(meterName))
+}
+
+// newSocialMetrics builds the instruments on meter, falling back to a no-op
+// counter on the (rare) construction error.
+func newSocialMetrics(meter metric.Meter) *socialMetrics {
+	c, err := meter.Int64Counter("chat_messages_total",
+		metric.WithDescription("Per-game chat entries posted, labelled by kind (message/nudge)."))
+	if err != nil {
+		c, _ = noop.NewMeterProvider().Meter(meterName).Int64Counter("chat_messages_total")
+	}
+	return &socialMetrics{messages: c}
+}
+
+// SetMetrics installs the meter the social domain records to. It must be called
+// during startup wiring; the default is a no-op meter.
+func (svc *Service) SetMetrics(meter metric.Meter) {
+	if meter == nil {
+		return
+	}
+	svc.metrics = newSocialMetrics(meter)
+}
+
+// recordChat counts one posted chat entry of the given kind (message or nudge).
+func (m *socialMetrics) recordChat(ctx context.Context, kind string) {
+	m.messages.Add(ctx, 1, metric.WithAttributes(attribute.String("kind", kind)))
+}

backend/internal/social/metrics_test.go

@@ -0,0 +1,48 @@
+package social
+
+import (
+	"context"
+	"testing"
+
+	"go.opentelemetry.io/otel/attribute"
+	sdkmetric "go.opentelemetry.io/otel/sdk/metric"
+	"go.opentelemetry.io/otel/sdk/metric/metricdata"
+)
+
+// TestSocialMetrics records chat and nudge entries through a manual reader and
+// asserts chat_messages_total splits by the "kind" attribute.
+func TestSocialMetrics(t *testing.T) {
+	ctx := context.Background()
+	reader := sdkmetric.NewManualReader()
+	meter := sdkmetric.NewMeterProvider(sdkmetric.WithReader(reader)).Meter("test")
+	m := newSocialMetrics(meter)
+
+	m.recordChat(ctx, kindMessage)
+	m.recordChat(ctx, kindMessage)
+	m.recordChat(ctx, kindNudge)
+
+	var rm metricdata.ResourceMetrics
+	if err := reader.Collect(ctx, &rm); err != nil {
+		t.Fatalf("collect: %v", err)
+	}
+
+	got := map[string]int64{}
+	for _, sm := range rm.ScopeMetrics {
+		for _, md := range sm.Metrics {
+			if md.Name != "chat_messages_total" {
+				continue
+			}
+			sum, ok := md.Data.(metricdata.Sum[int64])
+			if !ok {
+				t.Fatalf("chat_messages_total is not an int64 sum")
+			}
+			for _, dp := range sum.DataPoints {
+				v, _ := dp.Attributes.Value(attribute.Key("kind"))
+				got[v.AsString()] += dp.Value
+			}
+		}
+	}
+	if got[kindMessage] != 2 || got[kindNudge] != 1 {
+		t.Errorf("chat_messages_total = %v, want message:2 nudge:1", got)
+	}
+}

backend/internal/social/social.go

@@ -76,6 +76,7 @@ type Service struct {
 	accounts *account.Store
 	games    GameReader
 	pub      notify.Publisher
+	metrics  *socialMetrics
 	now      func() time.Time
 }
 
@@ -87,6 +88,7 @@ func NewService(store *Store, accounts *account.Store, games GameReader) *Servic
 		accounts: accounts,
 		games:    games,
 		pub:      notify.Nop{},
+		metrics:  defaultSocialMetrics(),
 		now:      func() time.Time { return time.Now().UTC() },
 	}
 }