feat(exporter/prometheus): add default_aggregation parameter to PrometheusMetricReader

[Manvi2402]

Description

Add default_aggregation parameter to PrometheusMetricReader to allow configuring default aggregation per instrument kind, aligning with the Prometheus Exporter spec.

Fixes #5109

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Test A: Added unit test test_default_aggregation in test_prometheus_exporter.py
  that verifies custom aggregation is correctly passed to the MetricReader.

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[MikeGoldsmith]

This looks good, thanks @Manvi2402. I've left some things to tweak.

I think it's gonna conflict with @herin049's PR (#5118). Whoever's PR merges later will likely have a conflict to resolve.

[MikeGoldsmith]

Indentation is wrong, this won't get run.

Suggested change

	def test_default_aggregation(self):
	"""Test that default_aggregation parameter is passed to MetricReader."""
	custom_aggregation = {
	HistogramInstrument: ExplicitBucketHistogramAggregation(
	boundaries=[1.0, 5.0, 10.0]
	)
	}
	reader = PrometheusMetricReader(
	default_aggregation=custom_aggregation
	)
	self.assertEqual(
	reader._instrument_class_aggregation[
	HistogramInstrument
	].__class__,
	ExplicitBucketHistogramAggregation,
	)
	reader.shutdown()
	def test_default_aggregation(self):
	"""Test that default_aggregation parameter is passed to MetricReader."""
	custom_aggregation = {
	HistogramInstrument: ExplicitBucketHistogramAggregation(
	boundaries=[1.0, 5.0, 10.0]
	)
	reader = PrometheusMetricReader(
	default_aggregation=custom_aggregation
	)
	self.assertEqual(
	reader._instrument_class_aggregation[
	HistogramInstrument
	].__class__,
	ExplicitBucketHistogramAggregation,
	)
	reader.shutdown()

[Manvi2402]

Fixed the indentation and updated the test to use public API instead of internal attributes

[MikeGoldsmith]

Why call this default_aggregation here? Would be nice to keep consistent as preferred_aggregation.

[Manvi2402]

Renamed to preferred_aggregation for consistency.

[MikeGoldsmith]

Suggested change

	([#5109](https://github.com/open-telemetry/opentelemetry-python/issues/5109))
	([#5117](https://github.com/open-telemetry/opentelemetry-python/issues/5117))

[Manvi2402]

I think #5109 is the correct issue number here, #5117 is the PR number. Please correct me if I'm wrong.

[MikeGoldsmith]

We prefer the changelog to use the PR number, not the associated issue number. You're welcome to add the issue number to the PR description if it's not there already 🙂

[Manvi2402]

Thanks for clarifying. Updated the changelog to use the PR number and also renamed default_aggregation to preferred_aggregation in the changelog entry.