Prevent any further processing for this
ClientCallStreamObserver. No further messages
will be received. The server is informed of cancellations, but may not stop processing the
call. Cancelling an already
cancel()ed
ClientCallStreamObserver has no effect.
No other methods on this class can be called after this method has been called.
It is recommended that at least one of the arguments to be non-
null, to provide
useful debug information. Both argument being null may log warnings and result in suboptimal
performance. Also note that the provided information will not be sent to the server.