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

shared/management/client/client.go

@@ -20,6 +20,7 @@ type Client interface {
 	GetDeviceAuthorizationFlow(serverKey wgtypes.Key) (*proto.DeviceAuthorizationFlow, error)
 	GetPKCEAuthorizationFlow(serverKey wgtypes.Key) (*proto.PKCEAuthorizationFlow, error)
 	GetNetworkMap(sysInfo *system.Info) (*proto.NetworkMap, error)
+	GetServerURL() string
 	IsHealthy() bool
 	SyncMeta(sysInfo *system.Info) error
 	Logout() error

shared/management/client/grpc.go

@@ -45,6 +45,7 @@ type GrpcClient struct {
 	conn                  *grpc.ClientConn
 	connStateCallback     ConnStateNotifier
 	connStateCallbackLock sync.RWMutex
+	serverURL             string
 }
 
 // NewClient creates a new client to Management service
@@ -74,9 +75,15 @@ func NewClient(ctx context.Context, addr string, ourPrivateKey wgtypes.Key, tlsE
 		ctx:                   ctx,
 		conn:                  conn,
 		connStateCallbackLock: sync.RWMutex{},
+		serverURL:             addr,
 	}, nil
 }
 
+// GetServerURL returns the management server URL
+func (c *GrpcClient) GetServerURL() string {
+	return c.serverURL
+}
+
 // Close closes connection to the Management Service
 func (c *GrpcClient) Close() error {
 	return c.conn.Close()