add metrics

[BEvgeniyS]

This adds some Prometheus metrics to scrape

• Standard Go metrics
• Metrics shipped with kubewebhook framework
• Custom new metrics

Please see metrics.md for details

[codecov[bot]]

Codecov Report

Patch coverage: 54.05% and project coverage change: -1.79 :warning:

Comparison is base (0a01de7) 67.34% compared to head (ad6af3d) 65.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #361      +/-   ##
==========================================
- Coverage   67.34%   65.55%   -1.79%     
==========================================
  Files           6        6              
  Lines         392      421      +29     
==========================================
+ Hits          264      276      +12     
- Misses        103      119      +16     
- Partials       25       26       +1     

Impacted Files	Coverage Δ
pkg/secrets/kubernetes.go	55.55% <0.00%> (-2.14%)	:arrow_down:
pkg/webhook/image_swapper.go	60.78% <40.00%> (-1.47%)	:arrow_down:
pkg/webhook/image_copier.go	71.53% <64.00%> (-3.05%)	:arrow_down:

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