topgun/k8s: Prometheus charts cross-compatibility

[cirocosta]

The current Topgun test suite for K8S right now include a cross-compatibility test with prometheus, the stable chart, but it seems like prometheus-operator is another chart that is used a lot out there.

In light of helm/charts#9536, we noticed that there might be incompatibility issues between Prometheus and Prometheus-operator (that led to a revert in upstream) - having a test that attested the compatibility would've helped.

As acceptance, we should:

• be able to have metrics being queried in a pod-specific manner from prometheus servers from:
  • prometheus
  • prometheus-operator