code: func sendExtHandshake(conn *net.TCPConn) error {
  	data := append(
  		[]byte{EXTENDED, HANDSHAKE},
  		Encode(map[string]interface{}{
  			"m": map[string]interface{}{"ut_metadata": 1},
  		})...,
  	)
  
  	return sendMessage(conn, data)
  }
docs: // sendExtHandshake requests for the ut_metadata and metadata_size.