Customer Facing Statsbeat: Added logic for dropped item count

[rads-1996]

Description

Follow up PR to #41669. This PR includes changes for dropped 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

Adds logic for tracking and reporting dropped telemetry items alongside existing success metrics, including:

• Implementation of count_dropped_items, _item_drop_callback, and _get_drop_reason in CustomerStatsbeat.
• New utility functions categorize_exception_message and categorize_status_code for low-cardinality drop reasons.
• Expanded test suite in test_customer_statsbeat.py to validate dropped item counting.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/statsbeat/test_customer_statsbeat.py	Added tests and helpers for dropped item counts but missing import of _TYPE_MAP.
statsbeat/_customer_statsbeat.py	Introduced count_dropped_items, drop counters, and observation callbacks.
exporter/_utils.py	Added categorize_exception_message and categorize_status_code utilities.
exporter/_constants.py	Adjusted envelope name mapping for dependencies in _TYPE_MAP.

Comments suppressed due to low confidence (3)

sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_utils.py:404

• Consider adding a docstring describing the purpose, input, and output of categorize_exception_message for clarity and consistency with the rest of the utility functions.

def categorize_exception_message(exception_message: str) -> str:

sdk/monitor/azure-monitor-opentelemetry-exporter/tests/statsbeat/test_customer_statsbeat.py:46

• The test references _TYPE_MAP but it is not imported. Please add from azure.monitor.opentelemetry.exporter._constants import _TYPE_MAP (or the correct module) at the top of this test file.

            for envelope_name, telemetry_type in _TYPE_MAP.items()

sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_constants.py:170

• The _TYPE_MAP key for dependencies should match the actual envelope name with Data suffix. Replace _REMOTE_DEPENDENCY_ENVELOPE_NAME with _REMOTE_DEPENDENCY_ENVELOPE_DATA to correctly map RemoteDependencyData envelopes.

                _REMOTE_DEPENDENCY_ENVELOPE_NAME: _DEPENDENCY,

[rads-1996]

[lzchen]

Please add a CHANGELOG entry!