otelgrpc: Add metrics support for stats handler

[pellared]

I noticed that the stats handler misses the functionality you added here: #2700. Any reasons for that?

I was plan to implement the stats.handler step by step, cover the trace first, and then cover the metrics. Which is also convenient for review.
Do you think I should implement it in this pr or create another pr for metrics?
I'm open for both way. @pellared

During the SIG meetings we discussed that we would like to have a feature-parity before we deprecate interceptors as part of #3002 (review).

Still I think we can tackle it in a separate PR.

Are there any other features which are missing when comparing with the interceptors?

I think metric is the only missing feature, since I wrote a simple e2e test TestStatsHandler just like interceptors to make sure the trace related function is complete.
@pellared

Originally posted by @fatsheep9146 in #3002 (comment)

[pellared]

@fatsheep9146 Can you take this?

[pellared]

It would be good to solve #4323 before this or as part of this issue.

[fatsheep9146]

It would be good to solve #4323 before this or as part of this issue.

Ok, I will try to fix #4323, if no one is working on this.

[Sovietaced]

Ok, I will try to fix #4323, if no one is working on this.

I'm happy to work on this if it means implementing an rcpcServerDuration histogram alongside the existing tracing code.

[fatsheep9146]

Ok, I will try to fix #4323, if no one is working on this.

I'm happy to work on this if it means implementing an rcpcServerDuration histogram alongside the existing tracing code.

I'm already working on this issue now, and before this work is merged, the #4323 should also be fixed, I'm now fixing the #4323 first. @Sovietaced

I think you could help review and add more meaningful metric after this issue is done if you want.