feat: Add trace when evaluating feature flags

[kylejuliandev]

This PR

Add trace when evaluating feature flags. This is different from the TraceEnricherHook which enriches the current span or trace with an event, instead this more closely follows the semantic conventions: https://opentelemetry.io/docs/specs/semconv/feature-flags/feature-flags-events/. Also add OpenTelemetry logging to the sample application and ensure traces are always sampled.

Success tracwe

Error trace

Related Issues

Fixes #595
Fixes #541

Notes

Follow-up Tasks

How to test

[codecov]

Codecov Report

❌ Patch coverage is 97.95918% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.59%. Comparing base (ec94acf) to head (7e99375).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/OpenFeature/OpenFeatureActivitySource.cs	96.55%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #755      +/-   ##
==========================================
+ Coverage   93.47%   93.59%   +0.12%     
==========================================
  Files          68       70       +2     
  Lines        2956     3030      +74     
  Branches      351      374      +23     
==========================================
+ Hits         2763     2836      +73     
  Misses        135      135              
- Partials       58       59       +1     

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[gemini-code-assist]

Code Review

This pull request implements OpenTelemetry tracing for feature flag evaluations in the OpenFeature client, including the addition of an activity source and corresponding unit tests. The review feedback highlights several discrepancies with OpenTelemetry Semantic Conventions, suggesting the use of standard attribute names (e.g., 'feature_flag.provider_name', 'feature_flag.evaluation_reason', 'feature_flag.evaluation_value', 'feature_flag.variant') and recommending that span statuses be explicitly set to 'Error' when failures occur to improve telemetry accuracy.