chore: Use a thread-safe collection for the custom instrumentation ids that are tracked in AgentHealthReporter

[tippmar-nr]

Fixes #3487

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.79%. Comparing base (d6634c8) to head (99fc00d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3517      +/-   ##
==========================================
- Coverage   81.81%   81.79%   -0.03%     
==========================================
  Files         508      508              
  Lines       34228    34228              
  Branches     4040     4040              
==========================================
- Hits        28005    27998       -7     
- Misses       5255     5265      +10     
+ Partials      968      965       -3     

Flag	Coverage Δ
Agent	82.78% <100.00%> (-0.03%)	⬇️
Profiler	71.75% <ø> (ø)

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Files with missing lines	Coverage Δ
...elic/Agent/Core/AgentHealth/AgentHealthReporter.cs	91.13% <100.00%> (ø)

... and 3 files with indirect coverage changes

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