[Shared] Refactor EnvironmentVariableScope

[martincostello]

Changes

Copy from open-telemetry/opentelemetry-dotnet-contrib to support multiple variables without needing to allocator multiple scopes as I've found that need while working on #7168 and #7174.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.06%. Comparing base (9b45df9) to head (ed8d682).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7175      +/-   ##
==========================================
- Coverage   89.08%   89.06%   -0.03%     
==========================================
  Files         271      271              
  Lines       13073    13073              
==========================================
- Hits        11646    11643       -3     
- Misses       1427     1430       +3     

Flag	Coverage Δ
unittests-Project-Experimental	88.98% <ø> (-0.06%)	⬇️
unittests-Project-Stable	89.04% <ø> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 2 files with indirect coverage changes

[Copilot]

Pull request overview

Refactors the test-only EnvironmentVariableScope helper to support setting/restoring multiple environment variables in a single scope, and updates impacted tests to use the new factory-based API.

Changes:

• Replaced single-variable EnvironmentVariableScope ctor usage with EnvironmentVariableScope.Create(...) across several tests.
• Refactored EnvironmentVariableScope to track and restore multiple variables.
• Minor expression-bodied member cleanup in TracerProviderBuilderBaseTests.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/OpenTelemetry.Tests/EnvironmentVariableScope.cs	Refactors helper to support multiple env vars and adds Create factory overloads.
test/OpenTelemetry.Tests/Trace/TracerProviderBuilderBaseTests.cs	Switches env var scoping to Create and simplifies helper methods.
test/OpenTelemetry.Tests/Metrics/MeterProviderBuilderBaseTests.cs	Switches env var scoping to Create.
test/OpenTelemetry.Tests/Logs/LoggerProviderBuilderBaseTests.cs	Switches env var scoping to Create.
test/OpenTelemetry.Extensions.Hosting.Tests/OpenTelemetryMetricsBuilderExtensionsTests.cs	Switches env var scoping to Create for OTEL_SDK_DISABLED test.
test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/OtlpMetricsExporterTests.cs	Switches env var scoping to Create for env-var-based configuration test.

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