code: public static <ReqT, RespT> StreamObserver<ReqT> asyncBidiStreamingCall(
        ClientCall<ReqT, RespT> call, StreamObserver<RespT> responseObserver) {
      return asyncStreamingRequestCall(call, responseObserver, true);
    }
docs: Executes a bidirectional-streaming call.  The {@code call} should not be already started.
  After calling this method, {@code call} should no longer be used.
  
  @return request stream observer.