Add FlagKey as prometheus label in eval counter

[glutamatt]

usefull for human friendly dashboards

[codecov-commenter]

Codecov Report

Merging #55 (f9c2f02) into main (1694d8e) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   81.55%   81.51%   -0.04%     
==========================================
  Files          27       27              
  Lines        2174     2175       +1     
==========================================
  Hits         1773     1773              
- Misses        314      315       +1     
  Partials       87       87              

Impacted Files	Coverage Δ
pkg/handler/eval.go	86.41% <0.00%> (-0.36%)	⬇️
pkg/config/config.go	78.75% <100.00%> (ø)

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