stats/opentelemetry: Cleanup OpenTelemetry API's before stabilization

[zasweq]

This PR cleans up some OpenTelemetry API's before stabilization.

RELEASE NOTES:

• stats/opentelemetry: Introduce new APIs to enable OpenTelemetry instrumentation for metrics on servers and clients

[codecov]

Codecov Report

Attention: Patch coverage is 82.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 82.12%. Comparing base (967ba46) to head (7f99944).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
stats/metrics.go	70.96%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7874      +/-   ##
==========================================
+ Coverage   81.94%   82.12%   +0.17%     
==========================================
  Files         377      377              
  Lines       38120    38112       -8     
==========================================
+ Hits        31239    31298      +59     
+ Misses       5572     5520      -52     
+ Partials     1309     1294      -15     

Files with missing lines	Coverage Δ
experimental/stats/metricregistry.go	100.00% <100.00%> (ø)
internal/testutils/stats/test_metrics_recorder.go	74.00% <100.00%> (ø)
stats/opentelemetry/client_metrics.go	87.93% <ø> (ø)
stats/opentelemetry/opentelemetry.go	75.86% <100.00%> (ø)
stats/opentelemetry/server_metrics.go	89.37% <ø> (ø)
stats/metrics.go	70.96% <70.96%> (ø)

... and 25 files with indirect coverage changes

[dfawley]

LGTM, just some minor wording suggestions. Please check out the edits-as-commits and suggested changes.

[dfawley]

Suggested change

	// cardinality and could cause severe memory or performance problems. On
	// Client Side, to record the method name along with metric instead of
	// bucketing into "other pass a grpc.StaticMethodCallOption as a call option
	// into Invoke or NewStream.
	//
	// This only applies for server side metrics.
	// cardinality and could cause severe memory or performance problems.
	//
	// This only applies for server-side metrics. For clients, to record
	// the method name in the attributes, pass grpc.StaticMethodCallOption
	// to Invoke or NewStream. Note that when using protobuf generated
	// clients, this CallOption is included automatically.

[zasweq]

Thanks, done.

[dfawley]

Suggested change

	// OptionalLabels specifies a list of optional labels that will be included
	// on any metrics that have corresponding optional labels configured.
	// OptionalLabels specifies a list of optional labels to enable on any
	// metrics that support them.

[zasweq]

Yeah I couldn't figure out a way to tersely convey it's just the subset that are "supported" that are enabled. (i.e. it's not always the full set added optionally, but the ones that are actually also configured. Changed to your suggestion.