Use a Sink argument instead of returning a Stream for response streaming

[chebbyChefNEQ]

Feature Request

Love the project! Just one request on response streaming.

Most (because I haven't used all of them) gRPC implementation passes a sink/writer handle into the handler stub. In the stub implementation, users may call sink.send(response) or write.write(response) to stream data to the client. However, tonic makes the handler return a Resonse<RPCStream> type.

This has two issues

1. This usually leaks tokio::spawn handles; or that user needs to build special apparatus to keep track of these handles.
2. We exit Layers before stream is completed

Crates

I think just tonic?

Motivation

1. stream back from inside the handler, instead of a stray tokio task
2. able to have more control over streaming response in Layers. -- concurrent stream limits, response coalescing, etc.

Proposal

TBD

Alternatives

• maybe tonic runtime can take a join handle in the handler return value and wait for the task to finish before exiting the layers? Less powerful then Sink interface but solves the control issue in Layer