code: <RequestT, ResponseT> void streamRequest(
        RequestT requestT,
        ApiStreamObserver<ResponseT> responseObserverT,
        ServerStreamingCallable<RequestT, ResponseT> callable) {
      Preconditions.checkState(!closed, "Firestore client has already been closed");
      callable.serverStreamingCall(requestT, responseObserverT);
    }
docs: Request funnel for all unidirectional streaming requests.