Modify sdkstats manager

[rads-1996]

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull request overview

This PR extends the statsbeat system in azure-monitor-opentelemetry-exporter to allow SDKs/distros to register additional metric callbacks and have their observations included alongside the built-in statsbeat network metrics.

Changes:

• Add a callback registry and an _iter_extra_observations helper to safely aggregate extra observations from registered callbacks.
• Add StatsbeatManager.add_metric_callback for registering per-metric callbacks (idempotent per callback instance).
• Update statsbeat network metric callbacks to append contributed observations, and add unit tests + changelog entry.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/statsbeat/_utils.py	Adds callback registry + iterator helper for contributed observations.
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/statsbeat/_manager.py	Adds StatsbeatManager.add_metric_callback API for registering callbacks.
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/statsbeat/_statsbeat_metrics.py	Appends extra observations to built-in network metrics callbacks.
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/statsbeat/test_metrics.py	Adds tests validating callback registration and observation aggregation behavior.
sdk/monitor/azure-monitor-opentelemetry-exporter/CHANGELOG.md	Documents the new callback capability under Features Added.