Add client metrics system with OpenTelemetry and VictoriaMetrics support

Implements a comprehensive client metrics system to track peer connection
stages and performance. The system supports multiple backend implementations
(OpenTelemetry, VictoriaMetrics, and no-op) and tracks detailed connection
stage durations from creation through WireGuard handshake.

Key changes:
- Add metrics package with pluggable backend implementations
- Implement OpenTelemetry metrics backend
- Implement VictoriaMetrics metrics backend
- Add no-op metrics implementation for disabled state
- Track connection stages: creation, semaphore, signaling, connection ready, and WireGuard handshake
- Move WireGuard watcher functionality to conn.go
- Refactor engine to integrate metrics tracking
- Add metrics export endpoint in debug server

client/server/debug.go

@@ -32,12 +32,20 @@ func (s *Server) DebugBundle(_ context.Context, req *proto.DebugBundleRequest) (
 		log.Warnf("failed to get latest sync response: %v", err)
 	}
 
+	var clientMetrics debug.MetricsExporter
+	if s.connectClient != nil {
+		if engine := s.connectClient.Engine(); engine != nil {
+			clientMetrics = engine.GetClientMetrics()
+		}
+	}
+
 	bundleGenerator := debug.NewBundleGenerator(
 		debug.GeneratorDependencies{
 			InternalConfig: s.config,
 			StatusRecorder: s.statusRecorder,
 			SyncResponse:   syncResponse,
 			LogFile:        s.logFile,
+			ClientMetrics:  clientMetrics,
 		},
 		debug.BundleConfig{
 			Anonymize:         req.GetAnonymize(),