Exempt specific GenAI attributes values from custom properties truncation

[rads-1996]

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.
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[Copilot]

Pull request overview

This PR updates Azure Monitor OpenTelemetry exporter custom-properties handling to avoid truncating values for a specific allowlist of GenAI-related attribute keys.

Changes:

• Add a _GEN_AI_ATTRIBUTES allowlist and exempt those keys’ values from the 64KiB truncation.
• Update _filter_custom_properties truncation logic to apply truncation only to non-GenAI keys.
• Update unit tests and documentation/changelog entries related to custom properties truncation.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_constants.py	Introduces _GEN_AI_ATTRIBUTES allowlist of GenAI attribute keys.
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_utils.py	Updates _filter_custom_properties to bypass truncation for allowlisted GenAI keys.
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/test_utils.py	Replaces env-var-based truncation tests with GenAI allowlist and mixed-key truncation tests.
sdk/monitor/azure-monitor-opentelemetry-exporter/README.md	Removes documentation for the truncation env var.
sdk/monitor/azure-monitor-opentelemetry-exporter/CHANGELOG.md	Adds an entry about GenAI truncation exemption (but needs reconciliation with other truncation entries).

Comments suppressed due to low confidence (1)

sdk/monitor/azure-monitor-opentelemetry-exporter/CHANGELOG.md:18

• The unreleased changelog entries around custom properties truncation are now inconsistent with the implementation in _filter_custom_properties: it no longer supports AZURE_MONITOR_DISABLE_CUSTOM_DIMENSIONS_LIMIT, yet the changelog still claims an env var exists, and it also includes an entry saying truncation was removed. Please reconcile these entries so they match the actual shipped behavior (and move any behavior removal to Breaking Changes if applicable).

- Exempt specific GenAI attributes values from custom properties truncation
  ([#45596](https://github.com/Azure/azure-sdk-for-python/pull/45596))
- Add environment variable to disable/enable custom properties truncation
  ([#45479](https://github.com/Azure/azure-sdk-for-python/pull/45479))
- Fix io counters import issue in performance counters
  ([#45286](https://github.com/Azure/azure-sdk-for-python/pull/45286))
- Remove custom properties truncation

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[JacksonWeber]

LGTM