Remove userAgent tag from HttpClient activity

[vishweshbankwar]

Fixes #
Design discussion issue #

Changes

User Agent is an opt-in tag so removing it https://github.com/open-telemetry/semantic-conventions/blob/main/docs/http/http-spans.md#http-client and it was also a bug to use older http.user_agent tag instead of user_agent.original

Merge requirement checklist

• CONTRIBUTING guidelines followed (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

Merging #5110 (ddd8c92) into main (61c6c74) will increase coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5110      +/-   ##
==========================================
+ Coverage   83.13%   83.15%   +0.01%     
==========================================
  Files         296      296              
  Lines       12318    12314       -4     
==========================================
- Hits        10241    10240       -1     
+ Misses       2077     2074       -3     

Flag	Coverage Δ
unittests	83.15% <ø> (+0.01%)	⬆️

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

Files	Coverage Δ
...tp/Implementation/HttpHandlerDiagnosticListener.cs	70.86% <ø> (-0.89%)	⬇️

... and 8 files with indirect coverage changes

[cijothomas]

Do we plan to provide an option to opt-in as well? Or just removal for now, and asking to use Enrich to get user-agent?

[vishweshbankwar]

Do we plan to provide an option to opt-in as well? Or just removal for now, and asking to use Enrich to get user-agent?

Would suggest Enrich for any additional attributes.

[cijothomas]

Do we plan to provide an option to opt-in as well? Or just removal for now, and asking to use Enrich to get user-agent?

Would suggest Enrich for any additional attributes.

That would be reasonable approach, as long we know dotnet/runtime#93019 is committed for .NET 9. Otherwise, its better to expose APIs for opt-in fields.
For unblocking stable release, its okay to do just-the-removal, but with a clear documentation on how users can self-mitigate.