Add support for OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION

[rajkumar-rangaraj]

Fixes #4153

Changes

This PR implements support for the OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION environment variable as specified in the OpenTelemetry specification.

Key Implementation Details

• Internal API only: All new types and properties are marked internal to avoid expanding the public API surface
• Spec-compliant parsing: Uses direct string comparison (case-insensitive) to validate against spec-defined snake_case values:
  • explicit_bucket_histogram (default)
  • base2_exponential_bucket_histogram
• View precedence: Explicit views configured via AddView take precedence over the default aggregation setting
• Applied in both code paths: Default aggregation is applied whether or not views are configured, ensuring consistent behavior

Implementation Components

1. MetricReaderHistogramAggregation (internal enum): Represents the two valid histogram aggregation types
2. MetricReaderOptions.DefaultHistogramAggregation (internal property): Stores the configured default
3. OtlpServiceCollectionExtensions: Parses the environment variable/configuration and sets the property
4. MetricReaderExt.CreateDefaultHistogramConfiguration: Helper method that applies the default to histogram instruments
5. Test coverage: 11 new tests covering environment variables, IConfiguration, view overrides, and edge cases

Testing

All existing tests pass (524/528), with 4 unrelated failures in other areas. New tests added:

• Configuration-based parsing
• Environment variable parsing
• Application to histogram instruments
• View override behavior
• Invalid value handling

Documentation

• Updated README.md with environment variable documentation
• Updated CHANGELOG.md with feature description

Merge requirement checklist

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

[codecov]

Codecov Report

❌ Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.77%. Comparing base (fe9e62d) to head (860765c).
⚠️ Report is 11 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...rc/OpenTelemetry/Metrics/Reader/MetricReaderExt.cs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6778      +/-   ##
==========================================
+ Coverage   86.69%   86.77%   +0.08%     
==========================================
  Files         262      262              
  Lines       12320    12350      +30     
==========================================
+ Hits        10681    10717      +36     
+ Misses       1639     1633       -6     

Flag	Coverage Δ
unittests-Project-Experimental	86.42% <96.77%> (-0.22%)	⬇️
unittests-Project-Stable	86.68% <96.77%> (-0.01%)	⬇️
unittests-Solution	86.75% <96.77%> (+0.12%)	⬆️
unittests-UnstableCoreLibraries-Experimental	86.07% <ø> (ø)

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

Files with missing lines	Coverage Δ
.../Implementation/OtlpServiceCollectionExtensions.cs	100.00% <100.00%> (ø)
src/OpenTelemetry/Metrics/Reader/MetricReader.cs	89.47% <100.00%> (+0.07%)	⬆️
...penTelemetry/Metrics/Reader/MetricReaderOptions.cs	100.00% <100.00%> (ø)
...rc/OpenTelemetry/Metrics/Reader/MetricReaderExt.cs	92.00% <92.85%> (+0.03%)	⬆️

... and 7 files with indirect coverage changes