opentelemetry-instrumentation-system-metrics: add OTEL_PYTHON_SYSTEM_METRICS_EXCLUDED_METRICS environment variable

[herin049]

Description

Implements #3954 to add support for the OTEL_PYTHON_SYSTEM_METRICS_EXCLUDED_METRICS environment variable in order to selectively exclude certain metrics from being exported.

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?

Several test scenarios have been added to test the new environment variable configuration option.

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

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

[sfozz]

Can confirm that this worked as expected.