Add user agent customization

[meijeran]

Fixes #6496

Design discussion issue #

Changes

Please provide a brief description of the changes here.

This pull request enhances the configuration of the User-Agent HTTP header in the OtlpExporterOptions class, allowing users to prepend custom product identifiers to the default exporter information. It introduces a new property for customization, updates header construction logic, and adds comprehensive tests to ensure correct behavior and compliance with HTTP standards.

User-Agent customization improvements:

• Added a public UserAgentProductIdentifier property to OtlpExporterOptions that allows users to specify a custom product identifier to be prepended to the default User-Agent header. The header updates dynamically when this property changes.
• Modified the logic for building the User-Agent string to include the custom identifier, ensuring proper formatting and RFC 7231 compliance (space-separated tokens).
• Changed StandardHeaders from a static readonly array to a static property backed by a private field, so it reflects the current user agent identifier. [1] [2]

Testing enhancements:

• Added multiple unit tests in OtlpExporterOptionsTests.cs to verify the default and custom behaviors of the UserAgentProductIdentifier property, including handling of empty/whitespace values, correct header formatting, and compliance with HTTP standards.

Merge requirement checklist

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

[martincostello]

Looks OK to me now - the maintainers just need to be happy with how it presents as a new public API.

[rajkumar-rangaraj]

@meijeran The directions look good. Could you please resolve the build issue? Also, please make sure not to include any unrelated changes in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.81%. Comparing base (b0b4211) to head (c89b979).
⚠️ Report is 15 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6686      +/-   ##
==========================================
+ Coverage   86.77%   86.81%   +0.04%     
==========================================
  Files         258      258              
  Lines       11990    11993       +3     
==========================================
+ Hits        10404    10412       +8     
+ Misses       1586     1581       -5     

Flag	Coverage Δ
unittests-Project-Experimental	86.67% <100.00%> (-0.10%)	⬇️
unittests-Project-Stable	86.74% <100.00%> (+0.06%)	⬆️

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	99.11% <100.00%> (+0.02%)	⬆️
...TelemetryProtocol/OtlpExporterOptionsExtensions.cs	98.95% <100.00%> (ø)

... and 2 files with indirect coverage changes

[github-actions]

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