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.

pkg/telemetry/telemetry_test.go

@@ -0,0 +1,84 @@
+package telemetry
+
+import (
+	"context"
+	"testing"
+	"time"
+)
+
+// TestConfigValidate covers the supported and rejected exporter selections.
+func TestConfigValidate(t *testing.T) {
+	for _, exp := range []string{ExporterNone, ExporterStdout, ExporterOTLP} {
+		c := DefaultConfig("svc")
+		c.TracesExporter = exp
+		c.MetricsExporter = exp
+		if err := c.Validate(); err != nil {
+			t.Errorf("exporter %q must be valid: %v", exp, err)
+		}
+	}
+	bad := DefaultConfig("svc")
+	bad.TracesExporter = "prometheus"
+	if err := bad.Validate(); err == nil {
+		t.Error("unsupported exporter must be rejected")
+	}
+	if err := DefaultConfig("").Validate(); err == nil {
+		t.Error("empty service name must be rejected")
+	}
+}
+
+// TestNewNoneAndStdout builds the providers with the none and stdout exporters,
+// starts the runtime metrics and shuts them down.
+func TestNewNoneAndStdout(t *testing.T) {
+	for _, exp := range []string{ExporterNone, ExporterStdout} {
+		cfg := DefaultConfig("svc")
+		cfg.TracesExporter = exp
+		cfg.MetricsExporter = exp
+		rt, err := New(context.Background(), cfg)
+		if err != nil {
+			t.Fatalf("New(%q): %v", exp, err)
+		}
+		if rt.TracerProvider() == nil || rt.MeterProvider() == nil {
+			t.Errorf("New(%q): nil provider", exp)
+		}
+		if err := rt.StartRuntimeMetrics(); err != nil {
+			t.Errorf("StartRuntimeMetrics(%q): %v", exp, err)
+		}
+		if err := rt.Shutdown(context.Background()); err != nil {
+			t.Errorf("Shutdown(%q): %v", exp, err)
+		}
+	}
+}
+
+// TestNewOTLPConstructs verifies the otlp exporter selection builds providers
+// without a reachable collector (the gRPC exporters dial lazily). Shutdown is
+// best-effort: with no collector the final export may error, so it is bounded and
+// its result ignored.
+func TestNewOTLPConstructs(t *testing.T) {
+	cfg := DefaultConfig("svc")
+	cfg.TracesExporter = ExporterOTLP
+	cfg.MetricsExporter = ExporterOTLP
+	rt, err := New(context.Background(), cfg)
+	if err != nil {
+		t.Fatalf("New(otlp): %v", err)
+	}
+	if rt.TracerProvider() == nil || rt.MeterProvider() == nil {
+		t.Error("New(otlp): nil provider")
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+	_ = rt.Shutdown(ctx)
+}
+
+// TestNilRuntime checks the nil-receiver fallbacks used before initialisation.
+func TestNilRuntime(t *testing.T) {
+	var rt *Runtime
+	if rt.TracerProvider() == nil {
+		t.Error("nil runtime must fall back to the global tracer provider")
+	}
+	if rt.MeterProvider() == nil {
+		t.Error("nil runtime must fall back to the global meter provider")
+	}
+	if err := rt.Shutdown(context.Background()); err != nil {
+		t.Errorf("nil runtime Shutdown: %v", err)
+	}
+}