Refactor prometheus leader helper

[aledbf]

What this PR does / why we need it:

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

[codecov-io]

Codecov Report

Merging #4251 into master will increase coverage by 0.09%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4251      +/-   ##
==========================================
+ Coverage    57.9%   57.99%   +0.09%     
==========================================
  Files          87       87              
  Lines        6544     6533      -11     
==========================================
  Hits         3789     3789              
+ Misses       2324     2313      -11     
  Partials      431      431

Impacted Files	Coverage Δ
internal/ingress/controller/nginx.go	29.12% <ø> (+0.39%)	⬆️
internal/ingress/controller/controller.go	46.62% <0%> (+0.11%)	⬆️

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[ElvinEfendi]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi

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