internal/stats: Moved Labels to experimental/stats package

[seth-epps]

Related to #8446

This follows up on the discussion here

By moving Labels and the associated Get/Set functions to the exported experimental/stats package we give client/server implementations of stats.Handler access to telemetry labels, otherwise only accessible if instrumented using an OpenTelemetry provider.

With this change, a handler implementation can start tracking telemetry labels by calling SetLabels in TagRPC/Tag Conn

RELEASE NOTES:

• internal/stats: Move Labels to experimental/stats package

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: seth-epps / name: Seth Epps (ea3db1c)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.02%. Comparing base (58d4b2b) to head (ea3db1c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8671      +/-   ##
==========================================
+ Coverage   81.76%   83.02%   +1.25%     
==========================================
  Files         417      417              
  Lines       40806    32296    -8510     
==========================================
- Hits        33367    26815    -6552     
+ Misses       6057     4097    -1960     
- Partials     1382     1384       +2     

Files with missing lines	Coverage Δ
experimental/stats/labels.go	100.00% <ø> (ø)
internal/stats/stats.go	100.00% <ø> (ø)
internal/xds/balancer/clusterimpl/picker.go	97.14% <100.00%> (+0.17%)	⬆️
stats/opentelemetry/client_metrics.go	88.11% <100.00%> (+1.44%)	⬆️

... and 359 files with indirect coverage changes

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[eshitachandwani]

LGTM. Deferring to @easwars for another review.

[dfawley]

I believe the decision to make these internal was made deliberately for cross-language parity reasons. So we might not want to do this without checking with the other languages first.

Since we need a cross language discussion before making this change.