[Instrumentation.EventCounters] Avoid race condition and performance trap in EventCountersMetrics

[MihaZupan]

This PR fixes two issues with the current implementation:

• Uses EventLevel.Critical instead of EventLevel.LogAlways
  • Counters don't require a specific log level to work. Depending on the EventSource implementor, this change can avoid a lot of overhead (preparing and serializing event arguments, potentially taking more expensive code paths).
• Avoids a race condition when initializing the EventCountersMetrics instance
  • As OnEventSourceCreated can run before the constructor is done, you can run into a situation where you fail to enable an EventSource instance. A simple lock avoids this race.
  • Example course of events:
    • Thread A calls into OnEventSourceCreated and sees options aren't set yet
    • Thread B executes the constructor, processing any entries in preInitEventSources
    • Thread A adds the EventSource into preInitEventSources
    • Nothing will process and enable that EventSource instance

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: MihaZupan / name: Miha Zupan (a222ac3)

[utpilla]

@MihaZupan Thanks for sending this PR! Could you please fix the build errors?

[codecov]

Codecov Report

Merging #703 (d3a5c03) into main (841e6e5) will decrease coverage by 0.22%.
The diff coverage is 100.00%.

❗ Current head d3a5c03 differs from pull request most recent head 7c1f538. Consider uploading reports for the commit 7c1f538 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #703      +/-   ##
==========================================
- Coverage   77.89%   77.66%   -0.23%     
==========================================
  Files         175      175              
  Lines        5297     5310      +13     
==========================================
- Hits         4126     4124       -2     
- Misses       1171     1186      +15     

Impacted Files	Coverage Δ
...trumentation.EventCounters/EventCountersMetrics.cs	82.75% <100.00%> (+1.98%)	⬆️
...orter.Geneva/MsgPackExporter/MsgPackLogExporter.cs	87.94% <0.00%> (-5.81%)	⬇️
...ry.Exporter.Geneva/Metrics/GenevaMetricExporter.cs	61.00% <0.00%> (-2.22%)	⬇️
....Exporter.Geneva/GenevaExporterHelperExtensions.cs	83.33% <0.00%> (+25.00%)	⬆️
...orter.Geneva/Internal/ServiceProviderExtensions.cs	100.00% <0.00%> (+100.00%)	⬆️

[mic-max]

LGTM Thanks!