@Override
            public void onCompleted() {
                responseObserver.onNext(StreamingInputCallResponse.newBuilder()
                                                                  .setAggregatedPayloadSize(
                                                                          totalPayloadSize).build());
                responseObserver.onCompleted();
            }
Waits until we have received all of the request messages and then returns the aggregate payload
size for all of the received requests.
grpc/src/test/java/com/linecorp/armeria/internal/common/grpc/TestServiceImpl.java