Prometheus exporter: Add default aggregation config option

[ArthurSens]

Description

Add default_aggregation to PrometheusMetricReader, allowing the Prometheus exporter to configure the MetricReader default aggregation as a function of instrument kind.
This follows the Prometheus exporter spec section being stabilized in open-telemetry/opentelemetry-specification#5113.

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?

• uv run tox -e py312-test-opentelemetry-exporter-prometheus -- exporter/opentelemetry-exporter-prometheus/tests/test_prometheus_exporter.py::TestPrometheusMetricReader::test_constructor_default_aggregation
• uv run ruff check exporter/opentelemetry-exporter-prometheus/src/opentelemetry/exporter/prometheus/__init__.py exporter/opentelemetry-exporter-prometheus/tests/test_prometheus_exporter.py

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

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Disclaimer: This PR was assisted by LLMs

[herin049]

Thanks for opening this PR! Unfortunately, we already have an open PR that addresses this: #5117.

[ArthurSens]

Oh, nice!