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.

gateway/internal/connectsrv/metrics.go

@@ -0,0 +1,43 @@
+package connectsrv
+
+import (
+	"context"
+	"time"
+
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/metric"
+	"go.opentelemetry.io/otel/metric/noop"
+)
+
+// meterName scopes the gateway edge's OpenTelemetry instruments.
+const meterName = "scrabble/gateway/edge"
+
+// serverMetrics holds the edge's operational instruments. It defaults to no-ops;
+// NewServer installs the real meter when one is supplied in Deps.
+type serverMetrics struct {
+	edge metric.Float64Histogram
+}
+
+// newServerMetrics builds the instruments on meter (nil selects a no-op meter),
+// falling back to a no-op histogram on the (rare) construction error.
+func newServerMetrics(meter metric.Meter) *serverMetrics {
+	if meter == nil {
+		meter = noop.NewMeterProvider().Meter(meterName)
+	}
+	h, err := meter.Float64Histogram("edge_request_duration",
+		metric.WithUnit("s"),
+		metric.WithDescription("Seconds to serve one Connect Execute call, by message type and result."))
+	if err != nil {
+		h, _ = noop.NewMeterProvider().Meter(meterName).Float64Histogram("edge_request_duration")
+	}
+	return &serverMetrics{edge: h}
+}
+
+// recordEdge records the duration of one Execute call labelled by message type and
+// outcome (ok, domain, unauthenticated, rate_limited, unknown_type or internal).
+func (m *serverMetrics) recordEdge(ctx context.Context, msgType, result string, start time.Time) {
+	m.edge.Record(ctx, time.Since(start).Seconds(), metric.WithAttributes(
+		attribute.String("message_type", msgType),
+		attribute.String("result", result),
+	))
+}