fix: don't panic when calling NewCtrlFuncMetrics() more than once

[pmalek]

What this PR does / why we need it:

It seems that we never tested calling dataplane.NewKongClient(), specifically more than once.

This PR fixes a panic which occurs when calling NewKongClient() twice. This is due to the fact that we call .MustRegister() on

kubernetes-ingress-controller/internal/metrics/prometheus.go

Line 111 in 0f55f29

metrics.Registry.MustRegister(controllerMetrics.ConfigPushCount, controllerMetrics.TranslationCount, controllerMetrics.ConfigPushDuration)

on prometheus RegistererGatherer.

Special notes for your reviewer:

Found when adding tests for #3421

[codecov]

Codecov Report

Base: 73.5% // Head: 73.4% // Decreases project coverage by -0.1% ⚠️

Coverage data is based on head (0f55f29) compared to base (4187223).
Patch coverage: 100.0% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3473     +/-   ##
=======================================
- Coverage   73.5%   73.4%   -0.1%     
=======================================
  Files        118     118             
  Lines      14065   14066      +1     
=======================================
- Hits       10345   10337      -8     
- Misses      3059    3066      +7     
- Partials     661     663      +2     

Impacted Files	Coverage Δ
internal/metrics/prometheus.go	100.0% <100.0%> (ø)
...nternal/controllers/gateway/tcproute_controller.go	70.0% <0.0%> (-2.0%)	⬇️
internal/dataplane/kongstate/service.go	66.0% <0.0%> (-1.3%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.