Allow optional monitoring of `/metrics` as well as `/public_metrics`

[JFlath]

What would you like to be added?

https://github.com/redpanda-data/helm-charts/blob/main/charts/redpanda/templates/servicemonitor.yaml

It would be nice to have an option to add an additional endpoint to hit /metrics here too.

Why is this needed?

Although this is generally not required, with our current setup it should be fairly painless to add as an option, which gives more chance for diagnosing issues in the small percentage of cases where this data would help.

JIRA Link: K8S-124