experimental/stats: Expose Telemetry Label Callback

[seth-epps]

Fixes #8682

Expose a new experimental API for registering a telemetry label callback function.

Some clients may not be instrumented with opentelemetry which restricts valuable information from being propagated to stats handlers. This gives clients the ability to collect otel labels by registering a label callback on the context and collecting the information themselves in their stats handlers.

RELEASE NOTES:

• experimental/stats: Expose Telemetry Label Callback

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.21%. Comparing base (49e224f) to head (12bf10e).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8877      +/-   ##
==========================================
+ Coverage   83.15%   83.21%   +0.05%     
==========================================
  Files         414      415       +1     
  Lines       32751    32765      +14     
==========================================
+ Hits        27235    27264      +29     
+ Misses       4096     4083      -13     
+ Partials     1420     1418       -2     

Files with missing lines	Coverage Δ
experimental/stats/telemetry.go	100.00% <100.00%> (ø)
internal/xds/balancer/clusterimpl/picker.go	95.23% <100.00%> (+0.11%)	⬆️

... and 20 files with indirect coverage changes

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