Add registry keyword argument to PrometheusMetricReader

[Manvi2402]

Description

Added a registry keyword argument to PrometheusMetricReader.init(), defaulting to the global REGISTRY. This allows users to pass a custom CollectorRegistry instance, which is useful for testing scenarios that require a fresh registry per test.
Note: The second part of the issue (set_meter_provider repeated calls) is not included in this PR.

Fixes #4713

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 test_custom_registry test in 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