Implement Customer Facing Statsbeat

[rads-1996]

Description

Implement customer facing statsbeat as a preview feature.

Packages impacted by this PR

@azure/monitor-opentelemetry-exporter

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

Implements preview support for customer-facing Statsbeat by integrating metric tracking into the exporter and local storage, and adding unit/integration tests.

• Hooks CustomerStatsbeatMetrics into the base exporter to count successful, dropped, and retried items.
• Enhances LocalFileStorage.put and get_items to emit Statsbeat metrics on storage errors, capacity, and expiry.
• Adds comprehensive tests for metrics collection and backend integration, plus updates constants and changelog.

Reviewed Changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
azure/monitor/opentelemetry/exporter/export/_base.py	Integrated customer statsbeat initialization and tracking in the transmission flow.
azure/monitor/opentelemetry/exporter/_storage.py	Extended storage to set and use customer_statsbeat_metrics for recording storage-related drops.
tests/statsbeat/test_customer_statsbeat_metrics.py	New unit tests covering success, drop, and retry counting methods.
tests/statsbeat/test_customer_statsbeat_integration.py	New integration tests verifying end-to-end metric export and failure handling.
tests/statsbeat/test_customer_statsbeat.py	Simplified initialization tests and disabled-mode behavior.
azure/monitor/opentelemetry/exporter/statsbeat/_customer_statsbeat_types.py	Defined new types and counters for customer Statsbeat.
azure/monitor/opentelemetry/exporter/statsbeat/init.py	Lazy-import setup for CustomerStatsbeatMetrics.
azure/monitor/opentelemetry/exporter/_constants.py	Added environment variable constant for enabling preview.
CHANGELOG.md	Documented the preview feature under 1.0.0b36.

[github-actions]

API Change Check

APIView identified API level changes in this PR and created the following API reviews

azure-monitor-opentelemetry-exporter

[rads-1996]

@lzchen I discovered that the envelope names and the base_type to determine the type of telemetry are a bit different. The envelope names are usually of the format - "Microsoft.ApplicationInsights.RemoteDependency", whereas the base_type is "RemoteDependencyData", that is with the suffix "data". I saw that although the envelope names are added to _constants.py, the base_type is hardcoded. I found this code from _exporter.py -

In the follow up PR, should I add the constants for base_type and use it in both customer statsbeat and the regular statsbeat. For now I have added the _REMOTE_DEPENDENCY_ENVELOPE_DATA in _constants.py.

[lzchen]

LGTM! Great job on this!