Read `OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION`

[kaylareopelle]

Spec requirement

The OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION env var (explicit_bucket_histogram/base2_exponential_bucket_histogram, default explicit_bucket_histogram) SHOULD be supported (sdk_exporters/otlp.md:43-70).

Current behavior

The default (ExplicitBucketHistogram, instrument/histogram.rb (permalink) default aggregation) is correct, but the env var itself is never read in exporter/otlp-metrics or metrics_sdk. A user can't switch the default to exponential histograms via configuration, only by explicitly registering a View.

Suggested fix

Read OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION in the OTLP exporter or metrics_sdk configurator, and use it to select the default histogram aggregation when no View overrides it.

Related rows in SPEC_COMPLIANCE_METRICS.md

OTLP-3

[Abhayanthk]

@kaylareopelle Happy to take this one. I’ll open a draft PR shortly and link it to this issue so you can follow my progress.