Add HttpClientFactory setting to OpAmpClientSettings

[stevejgordon]

Fixes #3588

Changes

Introduces a new HttpClientFactory setting in the OpAmpClientSettings class to allow customization of HttpClient instances for OpAMP HTTP transport. Updates PlainHttpTransport to utilize this factory, removing the HttpClientHandler and modifies OpAmpClient to pass settings to PlainHttpTransport.

This mostly copies the approach used for the OtlpExporterOptions in the main SDK.

• Adds a test to verify the use of the configured HttpClientFactory, including custom headers.
• Updates OpAmpFakeHttpServer to capture HTTP request headers and provide a method to retrieve them for testing.

Update CHANGELOG.md and PublicAPI.Unshipped.txt to document the new setting.

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

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.24%. Comparing base (48d18f6) to head (4503189).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/OpenTelemetry.OpAmp.Client/OpAmpClient.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3589      +/-   ##
==========================================
+ Coverage   71.22%   71.24%   +0.02%     
==========================================
  Files         442      442              
  Lines       17516    17515       -1     
==========================================
+ Hits        12475    12479       +4     
+ Misses       5041     5036       -5     

Flag	Coverage Δ
unittests-OpAmp.Client	72.24% <90.00%> (+0.69%)	⬆️

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

Files with missing lines	Coverage Δ
...ient/Internal/Transport/Http/PlainHttpTransport.cs	92.00% <100.00%> (+4.90%)	⬆️
...metry.OpAmp.Client/Settings/OpAmpClientSettings.cs	72.22% <100.00%> (+33.76%)	⬆️
src/OpenTelemetry.OpAmp.Client/OpAmpClient.cs	0.00% <0.00%> (ø)

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[Kielek]

In the issues description you have mentioned mutual TLS. Please consider this SDK PR open-telemetry/opentelemetry-dotnet#6343 in this context.

[Kielek]

@RassK taken longer break. He will be available next year. Is it fine to wait for him with this PR?

[stevejgordon]

@Kielek I have been chatting with @RassK on some blockers for us to start using the OpAmp client. This and one other (PR incoming today/tomorrow) are the main blockers I'm aware of immediately. Ideally we'd get these in sooner and potentially another release done so I can take that work further. Is that a possibility? Otherwise I'm blocked from working with this new feature. I'm hoping to them contribute in other areas as I identify other things we can improve.

[RassK]

@martincostello should be also quite familiar with the opamp part. Helped me to shape it. I think if he is fine with changes, don't wait for me.

@Kielek knows the issues of auto inst. side and it's contraints.

So I think it's quite safe to go forward a bit faster.

[Kielek]

LGTMRS