code: <RequestT, ResponseT> ApiStreamObserver<RequestT> streamRequest(
        ApiStreamObserver<ResponseT> responseObserverT,
        BidiStreamingCallable<RequestT, ResponseT> callable) {
      Preconditions.checkState(!closed, "Firestore client has already been closed");
      return callable.bidiStreamingCall(responseObserverT);
    }
docs: Request funnel for all bidirectional streaming requests.