code: func (nc *Conn) GetClientID() (uint64, error) {
  	nc.mu.RLock()
  	defer nc.mu.RUnlock()
  	if nc.isClosed() {
  		return 0, ErrConnectionClosed
  	}
  	if nc.info.CID == 0 {
  		return 0, ErrClientIDNotSupported
  	}
  	return nc.info.CID, nil
  }
docs: // GetClientID returns the client ID assigned by the server to which
  // the client is currently connected to. Note that the value may change if
  // the client reconnects.
  // This function returns ErrNoClientIDReturned if the server is of a
  // version prior to 1.2.0.