In cloud spanner Java client, we use manual flow control for the streaming rpcs here:
https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/GrpcSpannerRpc.java#L519

We would need this support in GAPIC code as well before we can change to use that. Specifically we want to manually request for more messages from the stream rather than the client automatically getting the next message.