Document metrics endpoint authentication

[adityathebe]

The /metrics endpoint now requires authentication by default.

To allow unauthenticated access, set:

metrics.auth.disabled=true

Related: flanksource/mission-control#2753

[DYTechWriter]

PR #512 documents this change.

The /metrics endpoint authentication requirement is now covered in the Prometheus integration page, including the metrics.auth.disabled=true Helm flag and a security note for network-restricted environments.