Customer Facing Statsbeat: Added logic for retry item count

[rads-1996]

Description

Follow up PR to #41669. This PR includes changes for retry item counts

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 Customer Facing Statsbeat to track and report retry item counts.

• Added RetryCode enum and RetryCodeType union to constants.
• Implemented count_retry_items, retry gauge, and _item_retry_callback in statsbeat telemetry.
• Updated tests and CHANGELOG to include retry counting logic.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
sdk/monitor/.../test_customer_statsbeat.py	Imported retry types and added retry test.
sdk/monitor/.../statsbeat/_customer_statsbeat.py	Added retry counters, methods, and callback.
sdk/monitor/.../exporter/_constants.py	Updated RetryCode enum and RetryCodeType.
sdk/monitor/.../CHANGELOG.md	Documented new retry item count feature.

Comments suppressed due to low confidence (2)

sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/statsbeat/_customer_statsbeat.py:122

• [nitpick] Consider adding a docstring to count_retry_items describing its parameters and behavior, analogous to the existing methods for success and drop counts.

    def count_retry_items(

sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/statsbeat/_customer_statsbeat.py:178

• There are no tests covering _item_retry_callback. Adding a unit test to verify the emitted Observation objects will ensure the callback logic is exercised.

    def _item_retry_callback(self, options: CallbackOptions) -> Iterable[Observation]: # pylint: disable=unused-argument

[rads-1996]