stats/opentelemetry: Add RLS Metrics e2e tests

[zasweq]

This PR adds e2e tests for RLS Metrics, with verifications of the OpenTelemetry metrics atoms emitted.

RELEASE NOTES:

• stats/opentelemetry: De-experimentalize OpenTelemetry instrumentation component and CSM Observability

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.30%. Comparing base (31ffeee) to head (832a2c6).
Report is 47 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7655      +/-   ##
==========================================
+ Coverage   81.73%   82.30%   +0.57%     
==========================================
  Files         361      367       +6     
  Lines       27817    28934    +1117     
==========================================
+ Hits        22735    23814    +1079     
- Misses       3871     3884      +13     
- Partials     1211     1236      +25     

see 47 files with indirect coverage changes

[easwars]

@zasweq : I'm not going to be able to get to this for at least a week. Please find another reviewer if you want it reviewed sooner. Thanks.

[dfawley]

As discussed offline: RLS's tests should go with RLS, not with OTel, and this PR is about testing RLS not OTel.

That also prevents the need to fork a copy of / move the testing helper code around.

[zasweq]

Moved to balancer/rls. Thanks.

[zasweq]

Fixed the go.mods due to moving it by deleting the stats/opentelemetry go.mod.

[dfawley]

This change makes us release the opentelemetry package and also CSM. We should make sure the release notes mention that. And make sure all the APIs contained in both are either marked as experimental or are believed to be stable.

[zasweq]

Due to merge conflicts from updating repository dependencies opened #7759.