How to provide backpressure when using ApiStreamObserver?

[meltsufin]

You can use the async GRPC client stub and pass in a ClientResponseObserver in place of the StreamObserver, which has an additional method called beforeStart that provides a ClientCallStreamObserver that has the request(count) method. However, I cannot find a similar request(count) method in ApiStreamObserver to provide backpressure.

[software-dov]

@vam-google @hzyi-google please triage or redirect.

[yihanzhen]

Please bump if necessary.