Support for extracting response headers for unary calls

[6harat]

Is your feature request related to a problem? Please describe.
We have a use-case where we want to simulate a unary call which returns context in its response headers which is required to be passed to subsequent unary calls. Based on our investigation, response Headers are not being captured in GrpcCallAction.ContinuingListener

    override def onHeaders(headers: Metadata): Unit = {}

And hence it would not be possible to extract that using this library without a code change.

Describe the solution you'd like
A utility method exposed in GrpcCheckSupport which will allow extracting the desired response header

Describe alternatives you've considered
No known alternatives found.

Additional context
NA

[phiSgr]

How in your application code do you capture the headers? The API in gRPC-Java for that, MetadataCapturingClientInterceptor, is quite clunky.

Also, why does the design use headers instead of trailers?

[6harat]

How in your application code do you capture the headers?

In our application code, we capture it using:

.withInterceptors(MetadataUtils.newCaptureMetadataInterceptor(headersCapture, trailersCapture))

Also, why does the design use headers instead of trailers?

The API was already created in the past with sending "some context" in response headers as the contract. It would be equally cumbersome to change that considering multiple other services depend upon that. For now we are just looking to add performance suite and found this to be a blocker to create our simulation.

I am happy to help here in terms of coding as well. It would be great if you could share the Contributing guideliens which I need to follow.

[phiSgr]

A new release should be available this weekend.

[6harat]

Possible to add a patch version to 0.10.x release.
Our performance test CI is still on scala 2.12. 🙏

I am assuming this commit: 1dc2cf0 refers to v0.10.1 (last publish of this series)

[6harat]

Possible to add a patch version to 0.10.x release.

This will require change in branching/tagging strategy as well for this repo. Not sure if that is a priority for this project at this time.
Otherwise, we are fine to maintain a fork for 0.10.x release on our side.

[6harat]

@phiSgr I have done the dev changes for the header extraction support in this PR: 6harat#1 pertaining to branch v1.10.x

Could u please have a look if it is correct.
Test case changes for ClientStreamingExample and StreamingExample are missing. I will look to add them shortly.

[6harat]

All test cases have been updated. The PR is code-complete from my side

[6harat]

@phiSgr Any updates on it?

[phiSgr]

Since you're maintaining your own fork, I'll take my time to update also the Kotlin API.

[6harat]

This will require change in branching/tagging strategy as well for this repo.

@phiSgr In that case, I am assuming the library won't be officially providing support for previous versions atleast for the time being now.