otelgrpc: Allow adding custom attributes to otelgrpc metrics

[inigohu]

Problem Statement

The otelgrpc instrumentation does not allow adding custom attributes to the metrics.

Proposed Solution

The otelhttp instrumentation, for example, provides a labeler for adding custom attributes.

[rehanpfmr]

@inigohu @MrAlias
Does this need to be updated on all instrumentations?
Let me know instrumentations which can be updated, I can take it.

[inigohu]

I see that the new otelgrpc.StatsHandler has been implemented. Would it be possible to include this feature? Maybe make gRPCContextKey and gRPCContext public to pass custom attributes? WDYT @pellared

[upalinski]

Any updates on this? Had to copy-paste stats handler and allow custom attributes to be passed outside.

[dmathieu]

I believe a similar approach to what's being done in net/http (the Labeler mentioned in the issue's description) would be fine, if you want to open a PR.

[duongcongtoai]

Hi, is there any update on this :D it is really useful for our usecase to tag extra labels to our grpc traffic