Merge branch 'jliu-gitaly-backup-metrics-doc' into 'master'

doc: Add section on Gitaly backup metrics

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138883



Merged-by: Evan Read <eread@gitlab.com>
Reviewed-by: Evan Read <eread@gitlab.com>
Co-authored-by: James Liu <jliu@gitlab.com>