[Prometheus] Add LabelsWithContext support to ClientMetrics to match ServerMetrics

[temporal-nick]

I noticed there wasn't a follow-on issue created from #445 / #758, so here it is. I use the Prometheus middleware to generate gRPC client and server metrics, and I jumped on WithLabelsFromContext as soon as it was available. I was surprised to find that it wasn't supported in the ClientMetrics, even though the UnaryClientInterceptor and StreamClientInterceptor accept WithLabelsFromContext as an option. Searching around a bit brought me to the linked issue and PR.

My app uses multiple gRPC clients that call the same gRPC service API in different ways, so I need labels to separate their metrics. I would also like the ability to derive labels from the calling Context, just like with the ServerMetrics.

[johanbrandhorst]

Hi, I'd be happy to consider a PR to make this symmetric with the server side. What can I do to help you bring this contribution?

[temporal-nick]

I need to check some priorities on my side. I should know whether I can work on this in about 2 weeks. Based on CONTRIBUTING, if I do have time it looks like I can just fork and make a PR?

[johanbrandhorst]

Yes, that's the general process, thank you. Try to limit any new API and definitely not breaking changes. Don't forget the docs, and new examples in the docs are particularly useful.

[temporal-nick]

Ok, I can start working on this later this month. Thanks for the info!