[OTLP] Add mTLS Support for OTLP Exporter

[sandy2008]

Fixes #2009
Design discussion issue #5918

Changes

This PR adds support for mutual TLS (mTLS) configuration to the OTLP exporter on .NET 8 and later. This allows users to specify certificate paths, enable validation, and inject custom server certificate validation logic.

Key changes include:

• OtlpMtlsOptions class with properties like ClientCertificatePath, ClientKeyPath, CaCertificatePath, EnableFilePermissionChecks, etc.
• OtlpMtlsHttpClientFactory for creating HttpClient instances configured for mTLS.
• OtlpMtlsCertificateManager for certificate loading, validation, and file permission checks.
• Extended event logging via OpenTelemetryProtocolExporterEventSource for mTLS events.
• Unit tests for configuration handling and certificate behavior.
• Initial integration and performance test stubs added.

mTLS support is only enabled for builds targeting NET8_0_OR_GREATER.

Merge requirement checklist

• [CONTRIBUTING](https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/CONTRIBUTING.md) 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

❌ Patch coverage is 58.54701% with 97 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.12%. Comparing base (d2f8e54) to head (cabc172).
⚠️ Report is 30 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...tocol/Implementation/OtlpMtlsCertificateManager.cs	47.28%	68 Missing ⚠️
...otocol/Implementation/OtlpMtlsHttpClientFactory.cs	55.93%	26 Missing ⚠️
...tation/OpenTelemetryProtocolExporterEventSource.cs	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6343      +/-   ##
==========================================
- Coverage   86.67%   86.12%   -0.56%     
==========================================
  Files         258      261       +3     
  Lines       11873    12099     +226     
==========================================
+ Hits        10291    10420     +129     
- Misses       1582     1679      +97     

Flag	Coverage Δ
unittests-Project-Experimental	86.02% <58.54%> (-0.50%)	⬇️
unittests-Project-Stable	85.91% <58.54%> (-0.36%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	99.25% <100.00%> (+0.16%)	⬆️
....Exporter.OpenTelemetryProtocol/OtlpMtlsOptions.cs	100.00% <100.00%> (ø)
...tation/OpenTelemetryProtocolExporterEventSource.cs	86.04% <62.50%> (+0.14%)	⬆️
...otocol/Implementation/OtlpMtlsHttpClientFactory.cs	55.93% <55.93%> (ø)
...tocol/Implementation/OtlpMtlsCertificateManager.cs	47.28% <47.28%> (ø)

... and 2 files with indirect coverage changes

[github-actions]

This PR was marked stale due to lack of activity and will be closed in 7 days. Commenting or pushing will instruct the bot to automatically remove the label. This bot runs once per day.

[sandy2008]

@rajkumar-rangaraj Hi :) Do you have time to review this PR?

[rajkumar-rangaraj]

@rajkumar-rangaraj Hi :) Do you have time to review this PR?

I'm on vacation and will be back only after a month.

[github-actions]

This PR was marked stale due to lack of activity and will be closed in 7 days. Commenting or pushing will instruct the bot to automatically remove the label. This bot runs once per day.

[sandy2008]

@rajkumar-rangaraj Hi :) Do you have time to review this PR?

I'm on vacation and will be back only after a month.

Got it, thank you! @rajkumar-rangaraj

[github-actions]

This PR was marked stale due to lack of activity and will be closed in 7 days. Commenting or pushing will instruct the bot to automatically remove the label. This bot runs once per day.

[sandy2008]

@alanwest Could you help change the requested change to approve~? Tyvm!

[github-actions]

This PR was marked stale due to lack of activity and will be closed in 7 days. Commenting or pushing will instruct the bot to automatically remove the label. This bot runs once per day.

[sandy2008]

@rajkumar-rangaraj Hi! Any updates?

[github-actions]

This PR was marked stale due to lack of activity and will be closed in 7 days. Commenting or pushing will instruct the bot to automatically remove the label. This bot runs once per day.

[github-actions]

This PR was marked stale due to lack of activity and will be closed in 7 days. Commenting or pushing will instruct the bot to automatically remove the label. This bot runs once per day.