Modify sdkstats manager

[rads-1996]

The corresponding PR in microsoft-opentelemetry - microsoft/opentelemetry-distro-python#191, currently the tests fail because they reference the additional_callbacks field which has been introduced in this PR.
Once the exporter is released and the latest version is pinned in the microsoft opentelemetry distro, we would be able to merge the PR.

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.

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• 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.

[lzchen]

What is this for?

[rads-1996]

I am getting some api inconsistency error in the CI/CD pipeline, trying to fix that. EDIT: So, I investigated the cause of this issue and looks like this PR - #47203 adds this new API consistency check which requires us to add the api.md and the metadata file. I believe this should be a one-time occurrence, unless the API is updated and we will have to push that change then.

[lzchen]

Let's add additional callbacks globals to state and have gettors / settors pattern just like the other global variables.

[lzchen]

We can then also make the additional callbacks as fields on the manager instead of accessing it directly from globals.

[rads-1996]

Addressed.