[OpenTelemetry] Use Regex source generator

[martincostello]

See #6849.

Changes

• Use [GeneratedRegex], where supported, for statically-known regular expressions.
• Apply analyzer suggestions to simplify and/or modernise code.

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 86.92%. Comparing base (496e4ac) to head (c9ac6c5).
⚠️ Report is 12 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6850      +/-   ##
==========================================
+ Coverage   86.90%   86.92%   +0.01%     
==========================================
  Files         263      263              
  Lines       12382    12378       -4     
==========================================
- Hits        10761    10760       -1     
+ Misses       1621     1618       -3     

Flag	Coverage Δ
unittests-Project-Experimental	86.71% <100.00%> (-0.01%)	⬇️
unittests-Project-Stable	86.83% <100.00%> (+0.01%)	⬆️

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

Files with missing lines	Coverage Δ
...nTelemetry/Internal/SelfDiagnosticsConfigParser.cs	88.13% <100.00%> (ø)
...lemetry/Metrics/Builder/MeterProviderBuilderSdk.cs	100.00% <100.00%> (+1.63%)	⬆️

... and 1 file with indirect coverage changes

[martincostello]

Using sizoscopeX, using the Regex source generator increases the code size in the test app compared to :

.mstat files generated by adding:

<PropertyGroup>
  <IlcGenerateDgmlFile>true</IlcGenerateDgmlFile>
  <IlcGenerateMstatFile>true</IlcGenerateMstatFile>
</PropertyGroup>

However, that project roots all the dependencies for OTel with TrimmerRootAssembly so all the code is retained, which isn't a fair comparison.

Will look at a comparison with a different app.

[martincostello]

Using this Lambda of mine (which just has traces), the size is increased from using the source generator:

Will try one more app that also has metrics, but I expect that will similarly be larger.

[martincostello]

Similar story with my personal website, code size goes up, mostly caused by the JSON paring Regexes. The metrics one adds 1kb.

[martincostello]

Opening for review/consideration purely from the analyzer resolution perspective, disregarding any native AoT code size impact.

[Copilot]

Pull request overview

This PR modernizes OpenTelemetry's regular expression usage by adopting the [GeneratedRegex] attribute for .NET 8.0+ targets, while maintaining backward compatibility with older frameworks through conditional compilation.

Changes:

• Replaced runtime-compiled regex instances with source-generated regex using [GeneratedRegex] for .NET 8.0+ targets
• Extracted regex patterns into const strings for better maintainability
• Applied code modernization (collection expressions, expression-bodied members)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/OpenTelemetry/Metrics/Builder/MeterProviderBuilderSdk.cs	Updated instrument name validation regex to use source generation on .NET 8.0+, modernized collection initializers and method syntax
src/OpenTelemetry/Internal/SelfDiagnosticsConfigParser.cs	Converted all four config parsing regex patterns to use source generation on .NET 8.0+, preserving fallback behavior for older frameworks

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