Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2427

This just enables the endpoint, which is somewhat helpful, but not
really enough to scrape them. Ideally, we'd be injecting these targets
into the Prometheus scrape config too.
For now, registering targets with Prometheus is very manual
(`matrix_prometheus_scraper_postgres_enabled`, `matrix_prometheus_scraper_hookshot_enabled`, ..).
This should be redone - e.g. a new `matrix_prometheus_scrape_config_jobs_auto` variable,
which is dynamically built in `group_vars/matrix_servers`.