Create end-to-end test to verify Prometheus operator metrics

[simonpasquier]

This is a follow-up of #6513 and #6525.

To avoid future regression on key operator metrics disappearing when we update dependencies, we need to add an end-to-end test.

Key metrics involve at least the ones used in the alerts mixin the but others can be included.

Originally posted by @simonpasquier in #6525 (comment)

[afzal442]

Hey @simonpasquier this should go into metrics_test.go new file to add an end-to-end test . Can we take an example of it https://github.com/kubernetes-sigs/controller-runtime/blob/67b27f27e514bd9ac4cf9a2d84dec089ece95bf7/pkg/metrics/client_go_adapter.go#L30-L55 to create test cases for prom-op metrics?