Fix praefect prometheus configuration
Ordering is important on .toml files, since once a section starts all
settings belong to that section until the next one begins. In Praefect's
case that meant we were putting prometheus_listen_addr
under the
auth
section, when it should be unscoped, which caused the settings to
be rendered incorrectly
Closes https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8327