code: public static <ReqT, RespT> ListenableFuture<RespT> futureUnaryCall(
        ClientCall<ReqT, RespT> call, ReqT req) {
      GrpcFuture<RespT> responseFuture = new GrpcFuture<>(call);
      asyncUnaryRequestCall(call, req, new UnaryStreamToFuture<>(responseFuture), false);
      return responseFuture;
    }
docs: Executes a unary call and returns a {@link ListenableFuture} to the response.  The
  {@code call} should not be already started.  After calling this method, {@code call} should no
  longer be used.
  
  @return a future for the single response message.