Update suggested value for the metrics address

Within GitLab its reserved at the new address which will influence the setup steps for users if they want to use Prometheus.

[ci skip]

/cc @maratkalibek

Supporting docs: https://github.com/prometheus/prometheus/wiki/Default-port-allocations

@zj Pages docs should also receive this update.

merged

changed milestone to %9.0

assigned to @marin

Pages MR: gitlab-pages!17 (merged)