prometheus: mimir optional integration

[NachoxMacho]

I've currently got prometheus remote writing to mimir, and would like to use the prometheus plugin to pull the metrics, but can't provider the mimir endpoint as the prometheus api is on the path /prometheus. I could probably run another query frontend just for headlamp, but that seems like overkill to just enable this one plugin.

Ideally I think we should be able to append a path to the service definition, but barring that able to pass a full url as an alternative in the case the prometheus instance isn't directly available inside the cluster.

[jerry-pond]

please also support basic auth for prometheus service