stats/opentelemetry: enable otel dial option to pass empty MetricOptions{}

[aranjans]

This PR addresses the issue where users cannot pass empty MetricOptions in the gRPC-Go OpenTelemetry implementation, leading to runtime errors. This allows user to pass empty MetricOptions which enhances flexibility for users who want to disable metrics collection without a full configuration.

Changes Made

• No-Op Metric Recorder: Introduced a NoOpMetricsRecorder to handle cases where metrics are not enabled.
• Test Coverage: Added tests to verify correct behavior with empty MetricOptions.

RELEASE NOTES:

• stats/opentelemetry: enable otel dial option to pass empty MetricOptions{}.

[codecov]

Codecov Report

Attention: Patch coverage is 79.41176% with 7 lines in your changes missing coverage. Please review.

Project coverage is 82.17%. Comparing base (56a14ba) to head (f5af085).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
stats/opentelemetry/opentelemetry.go	0.00%	5 Missing ⚠️
stats/opentelemetry/client_metrics.go	93.75%	1 Missing ⚠️
stats/opentelemetry/server_metrics.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7952      +/-   ##
==========================================
+ Coverage   82.09%   82.17%   +0.08%     
==========================================
  Files         379      381       +2     
  Lines       38261    38558     +297     
==========================================
+ Hits        31409    31686     +277     
- Misses       5549     5570      +21     
+ Partials     1303     1302       -1     

Files with missing lines	Coverage Δ
stats/opentelemetry/client_metrics.go	88.46% <93.75%> (+0.53%)	⬆️
stats/opentelemetry/server_metrics.go	91.56% <92.30%> (+2.19%)	⬆️
stats/opentelemetry/opentelemetry.go	73.33% <0.00%> (-2.53%)	⬇️

... and 32 files with indirect coverage changes

[arjan-bal]

I think the only changes needed may be to set the metrics and the MetricsProvider to no-op versions, rest of the changes can be reverted.

Suggested change

	if h.options.MetricsOptions.MeterProvider == nil {
	return
	}
	if h.options.MetricsOptions.MeterProvider == nil {
	h.clientMetrics.attemptStarted = noop.Int64Counter{}
	h.clientMetrics.attemptDuration = noop.Float64Histogram{}
	h.clientMetrics.attemptSentTotalCompressedMessageSize = noop.Int64Histogram{}
	h.clientMetrics.attemptRcvdTotalCompressedMessageSize = noop.Int64Histogram{}
	h.clientMetrics.callDuration = noop.Float64Histogram{}
	h.MetricsRecorder = &registryMetrics{}
	return
	}

[arjan-bal]

If we set the metrics in setClientMetrics to an empty set using stats.NewMetricSet(), we end up getting no-op implementations for the clientMetrics and registryMetrics. We can use this fact to unify the code paths for MeterProvider == nil and MeterProvider != nil.

[aranjans]

@arjan-bal I have updated the PR with suggested change above. Could you clarify this comment?

[arjan-bal]

With the current approach, the list of metrics (attemptStarted, attemptDuration, attemptSentTotalCompressedMessageSize etc.) is duplicated. If we set metrics to an empty set using stats.NewMetricSet(), the existing code will set every metric to the corresponding noop histogram, avoiding duplication.