app: add metric for enabled feature flags

[KaloyanTanev]

As per the title. The ticket explains the reasoning.

category: feature
ticket: #3979

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.56%. Comparing base (61c3490) to head (45f5fd2).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
app/featureset/featureset.go	0.00%	11 Missing ⚠️
app/metrics.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3994      +/-   ##
==========================================
- Coverage   53.58%   53.56%   -0.03%     
==========================================
  Files         224      224              
  Lines       37329    37343      +14     
==========================================
  Hits        20002    20002              
- Misses      15208    15222      +14     
  Partials     2119     2119              

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