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]

Looks good - thanks @Manvi2402 👍🏻

[herin049]

Can we do some additional verification here to actually verify that the boundaries specified above are respected?

[Manvi2402]

Done! Added verification to check that the custom boundaries [1.0, 5.0, 10.0] are present in the Prometheus output. Thanks for the suggestion.

[github-actions]

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[MikeGoldsmith]

@herin049 please can you re-review, @Manvi2402 made the change you suggested.