don't wrap conns with metrics if bwc is nil

p2p/net/swarm/swarm.go

@@ -109,9 +109,12 @@ func NewSwarm(ctx context.Context, listenAddrs []ma.Multiaddr,
 		return nil, err
 	}
 
-	wrap := func(c transport.Conn) transport.Conn {
+	var wrap func(c transport.Conn) transport.Conn
+	if bwc != nil {
+		wrap = func(c transport.Conn) transport.Conn {
 			return mconn.WrapConn(bwc, c)
 		}
+	}
 
 	s := &Swarm{
 		swarm:  ps.NewSwarm(PSTransport),