Promote overflow attribute from experimental to stable

[xiang17]

Fixes #5641. Design discussion issue #

Changes

Remove the OTEL_DOTNET_EXPERIMENTAL_METRICS_EMIT_OVERFLOW_ATTRIBUTE variable and make the feature enabled by default.

Merge requirement checklist

• [x] CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• [x] Unit tests added/updated
• [x] Appropriate CHANGELOG.md files updated for non-trivial changes
• [x] Changes in public API reviewed (if applicable)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.27%. Comparing base (6250307) to head (e4e21ca). Report is 360 commits behind head on main.

Files with missing lines	Patch %	Lines
src/OpenTelemetry/Metrics/AggregatorStore.cs	77.77%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5909      +/-   ##
==========================================
+ Coverage   83.38%   86.27%   +2.88%     
==========================================
  Files         297      260      -37     
  Lines       12531    11437    -1094     
==========================================
- Hits        10449     9867     -582     
+ Misses       2082     1570     -512     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.23% <80.00%> (?)
unittests-Project-Stable	86.21% <80.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry/Metrics/MeterProviderSdk.cs	93.38% <100.00%> (+5.54%)	:arrow_up:
src/OpenTelemetry/Metrics/Metric.cs	97.27% <ø> (+0.75%)	:arrow_up:
...rc/OpenTelemetry/Metrics/Reader/MetricReaderExt.cs	92.03% <ø> (ø)
src/OpenTelemetry/Metrics/AggregatorStore.cs	86.15% <77.77%> (+5.76%)	:arrow_up:

... and 233 files with indirect coverage changes