code: func (tc *serverTransportCredentials) ClientHandshake(ctx context.Context, addr string, rawConn net.Conn) (net.Conn, credentials.AuthInfo, error) {
  	return nil, nil, ClientHandshakeNopErr
  }
docs: // ClientHandshake is not implemented for a `serverTransportCredentials`, use
  // a `clientTransportCredentials` if you require `ClientHandshake`.