Simplify handler registration and updates prometheus

[aledbf]

What this PR does / why we need it:

Updates prometheus go client and k8s dependencies.

[antoineco]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [aledbf,antoineco]

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[codecov-io]

Codecov Report

Merging #2775 into master will decrease coverage by 0.28%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2775      +/-   ##
==========================================
- Coverage    47.8%   47.51%   -0.29%     
==========================================
  Files          74       74              
  Lines        5384     5400      +16     
==========================================
- Hits         2574     2566       -8     
- Misses       2485     2509      +24     
  Partials      325      325

Impacted Files	Coverage Δ
cmd/nginx/main.go	20.39% <0%> (-7.55%)	⬇️
internal/watch/file_watcher.go	80.76% <0%> (-3.85%)	⬇️

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