[docs] Prometheus monitoring label issue

[jmcx]

See https://triggermesh-community.slack.com/archives/C02GHUAQDCH/p1685085885165109?thread_ts=1684724222.983259&cid=C02GHUAQDCH for context.

Default prometheus operator install isn't compatible with our observability documentation.

Info: if you installed Prometheus with the operator, it uses a label selector for pod monitors. If you create a pod monitor that doesn't have those labels, prometheus will not pick it up. Solution is to either reconfigure the selector installed by Prometheus, which is:

Pod Monitor Selector:
Match Labels:
Release: kube-prometheus-stack

Or update the pod monitor to include label:

release=kube-prometheus-stack