Document how to provide certs for proxied /metrics endpoints

[paulfantom]

What is missing?
In cases where kube-prometheus adds kube-rbac-proxy in front of /metrics endpoints we set not to verify TLS certs. There should be a documented way on how to set insecureSkipVerify: true and provide certs.

Why do we need it?
To provide a better way to secure cluster.

Environment

• kube-prometheus version: latest

Anything else we need to know?:
Initial discussion: #453 (comment)

[paulfantom]

A part of #179

[brancz]

Been wanting to do this with a cert-manager setup for years, but never got around to it, would be really awesome if someone took the time for this!