Customer SDKStats: Rename metric names and set the environment variable on by default

[rads-1996]

Description

Customer SDKStats: Rename metric names and set the environment variable on by default

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]

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[Copilot]

Pull request overview

This PR updates the customer SDK stats feature to use a new environment variable convention (default-on via a ..._DISABLED flag) and renames the emitted metric names to their final, non-preview forms.

Changes:

• Replace APPLICATIONINSIGHTS_SDKSTATS_ENABLED_PREVIEW with APPLICATIONINSIGHTS_SDKSTATS_DISABLED and change is_customer_sdkstats_enabled() semantics so customer SDK stats are enabled by default unless explicitly disabled.
• Rename CustomerSdkStatsMetricName values from preview.item.*.count to Item_*_Count strings used when creating the observable gauges.
• Update all affected tests (base exporter, customer sdkstats utilities/manager/core tests) and the CHANGELOG entry to match the new behavior and naming.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/test_base_exporter.py	Adjusts environment setup in base exporter tests to use APPLICATIONINSIGHTS_SDKSTATS_DISABLED and reflect the default-on behavior.
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/test_base_customer_sdkstats.py	Switches customer SDK stats test setup/teardown to the new ..._DISABLED environment variable semantics.
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/customer_sdk_stats/test_utlities.py	Updates imports, env var handling, and is_customer_sdkstats_enabled tests to align with the disabled-flag and default-enabled logic.
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/customer_sdk_stats/test_manager.py	Uses the new ..._DISABLED flag in manager setup and disabled-path tests, ensuring manager status reflects the updated enable/disable semantics.
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/customer_sdk_stats/test_customer_sdkstats.py	Updates core customer SDK stats tests to enable/disable via APPLICATIONINSIGHTS_SDKSTATS_DISABLED.
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/statsbeat/customer/_utils.py	Changes is_customer_sdkstats_enabled() to read APPLICATIONINSIGHTS_SDKSTATS_DISABLED and default to enabled when unset.
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_constants.py	Replaces the *_ENABLED_PREVIEW constant with *_DISABLED and updates CustomerSdkStatsMetricName values to new metric names.
sdk/monitor/azure-monitor-opentelemetry-exporter/CHANGELOG.md	Documents the metric name rename and default-on behavior for customer SDK stats in the unreleased 1.0.0b47 section.