New method StreamObserver.complete(value) for unary RPC

[panchenko]

I think unary RPCs are used quite often, then in the service implementation we have to write 2 lines:

responseObserver.onNext(result);
responseObserver.onCompleted();

It seems logical to introduce a default method:

default void complete(V value) {
  onNext(value);
  onCompleted();
}

WDYT?

[sergiitk]

Thanks for the suggestion. We're going to discuss it with the team at our next API Stabilization meeting.

[ejona86]

API review notes (2024-04-09 (?) and 2024-04-18)

• This could be done through a utility function. Doesn't seem people have made utility functions for themselves for this. If we just added it as a utility function, could be added to “StreamObservers” (and un-deprecate the class)
• There's the option for Exception handling in the implementation - we don’t encourage try/catch around next in general, so not necessary for initial implementation
• Shouldn’t name it “complete”; that's ambiguous - “onNextAndCompleted” seemed better
• This is just added as syntactic sugar, users can use it if they want, but we don't need to encourage ti. We don't need to update the existing examples/docs to use it
• Approved as experimental. @panchenko, feel free to send a PR to add StreamObserver.onNextAndCompleted()