WIP: Update Prometheus documentation to caveat SSL/HSTS limitation

What does this MR do?

Updates the Prometheus documentation to clearly state that when SSL has been enabled on GitLab, access to Prometheus will not be possible due to HSTS.

Also updated to note that Prometheus and its exporters are unauthenticated.

Why was this MR needed?

This was not clearly documented, and so has caused confusion among administrators.

Does this MR meet the acceptance criteria?

This MR is just a documentation change.

What are the relevant issue numbers?

gitlab-ce#27069