Customer SDKStats: Preserve casing for metric names

[rads-1996]

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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[rads-1996]

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[Copilot]

Pull request overview

This PR fixes a bug where customer SDK stats metric names were not displaying with the correct casing in Azure Monitor. The issue occurred because OpenTelemetry normalizes metric names to lowercase, but there was no mapping to restore the proper Pascal case (e.g., "Item_Success_Count") when exporting to Azure Monitor.

Changes:

• Added a mapping dictionary to convert lowercase OpenTelemetry metric names to Pascal case display names for customer SDK stats
• Updated the CustomerSdkStatsMetricName enum values to use lowercase names (matching what OpenTelemetry produces) instead of Pascal case
• Applied the metric name mapping in the metrics exporter when processing customer SDK stats metrics

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_constants.py	Added metric name mapping dictionary and updated enum values to lowercase to match OpenTelemetry's normalization
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/export/metrics/_exporter.py	Added import and logic to apply customer SDK stats metric name mapping when exporting metrics
sdk/monitor/azure-monitor-opentelemetry-exporter/CHANGELOG.md	Added bug fix entry documenting the casing preservation fix