opentelemetry-sdk: make methods on FixedSizeExemplarReservoirABC thread safe

[herin049]

Description

Per the OpenTelemetry Specification all methods on ExemplarReservoir MUST be safe to be called concurrently.

ExemplarReservoir - all methods MUST be safe to be called concurrently.

This PR introduces a lock to synchronize methods calls on the FixedSizeExemplarReservoirABC class and all subclasses.

Fixes #5431

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 py314-test-opentelemetry-sdk

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