[Azure Monitor Exporter] Add custom resource support

[rajkumar-rangaraj]

• Add custom resource support
• We have test coverage for this feature
• Fixed flaky test - ValidatePerfCounterMetrics

[Copilot]

Pull Request Overview

This PR enables resource metrics export by default in the Azure Monitor OpenTelemetry Exporter by changing the behavior when the EXPORT_RESOURCE_METRIC environment variable is not set. Additionally, it addresses a flaky test in ValidatePerfCounterMetrics by implementing proper waiting logic for performance counter collection cycles.

Key Changes:

• Modified default behavior to export resource metrics when the environment variable is null or "true" (previously only "true")
• Fixed flaky test by adding SpinWait logic to properly wait for performance counter collection before assertions

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
ResourceExtensions.cs	Changed condition to enable resource metric export by default (when env var is null)
ResourceExtensionsTests.cs	Updated test assertions to account for new default behavior where null env var enables resource metrics
StandardMetricTests.cs	Added SpinWait logic to properly wait for performance counter collection cycle completion
CHANGELOG.md	Added entry documenting the new default behavior for resource metrics export