feat(metrics): add OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION support

[michaelbeaumont]

See #3374:

OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION — spec values are explicit_bucket_histogram, base2_exponential_bucket_histogram; no issue or PR

Changes

Add support for resolving OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION from the environment and threading it into exporters.

This adds a new method to the traits. If this is a problem it can certainly be replaced with a default implementation.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 72.25131% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.6%. Comparing base (f60125e) to head (0a50255).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-otlp/src/metric.rs	80.1%	22 Missing ⚠️
opentelemetry-sdk/src/metrics/pipeline.rs	59.2%	11 Missing ⚠️
opentelemetry-stdout/src/metrics/exporter.rs	0.0%	11 Missing ⚠️
opentelemetry-otlp/src/exporter/tonic/mod.rs	0.0%	6 Missing ⚠️
opentelemetry-sdk/src/metrics/exporter.rs	0.0%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3433     +/-   ##
=======================================
- Coverage   83.7%   83.6%   -0.1%     
=======================================
  Files        126     126             
  Lines      25386   25561    +175     
=======================================
+ Hits       21255   21380    +125     
- Misses      4131    4181     +50     

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[cijothomas]

give default implementation, else this is a breaking change.