Fix customer SDK stats metrics to display drop code and retry code enum values without the prefix

[rads-1996]

Description

Fix the dropcode and retrycode naming. Only shows the actual dropcode and retrycode without the prefix.

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.

[rads-1996]

Dropped Count -

Retry Count -

[Copilot]

Pull request overview

This PR fixes the display format of drop code and retry code enum values in customer SDK stats metrics by ensuring that only the string value (e.g., "CLIENT_READONLY") is used in metrics attributes, rather than the full enum representation.

Changes:

• Modified _item_drop_callback to use .value on DropCode enums when setting the "drop.code" attribute
• Modified _item_retry_callback to use .value on RetryCode enums when setting the "retry.code" attribute
• Updated CHANGELOG with bug fix entry

Reviewed changes

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

File	Description
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/statsbeat/customer/_manager.py	Added conditional logic to extract string values from enum types for drop and retry codes
sdk/monitor/azure-monitor-opentelemetry-exporter/CHANGELOG.md	Documented the bug fix in the unreleased version section