[OpAMP] Send custom messages

[RassK]

Follow up to #3681

Changes

Enables agent to send custom messages.

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 85.71429% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.02%. Comparing base (41eb903) to head (3674e9e).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ry.OpAmp.Client/Internal/OpAmpClientEventSource.cs	57.14%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3809   +/-   ##
=======================================
  Coverage   72.01%   72.02%           
=======================================
  Files         447      447           
  Lines       17792    17834   +42     
=======================================
+ Hits        12813    12845   +32     
- Misses       4979     4989   +10     

Flag	Coverage Δ
unittests-OpAmp.Client	81.69% <85.71%> (-0.31%)	⬇️

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

Files with missing lines	Coverage Δ
...penTelemetry.OpAmp.Client/Internal/FrameBuilder.cs	75.96% <100.00%> (+3.13%)	⬆️
...Telemetry.OpAmp.Client/Internal/FrameDispatcher.cs	94.02% <100.00%> (+1.57%)	⬆️
src/OpenTelemetry.OpAmp.Client/OpAmpClient.cs	88.88% <100.00%> (+0.42%)	⬆️
...ry.OpAmp.Client/Internal/OpAmpClientEventSource.cs	60.65% <57.14%> (-1.05%)	⬇️

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[stevejgordon]

A few minor suggestions.

[stevejgordon]

For consistency, should this message be: Failed to send a custom message message? This currently reads as if more than one custom message failed?

[RassK]

The intent was Failed to send a custom message (without the typo). Failed to send a custom message message is definitely a more consistent way but just sounds weird. Any preferences here?

[stevejgordon]

I'm fine with Failed to send a custom message.