Transitioning from push-based to pull-based monitoring with
Prometheus/collectd. The *write_prometheus* plugin will be installed on
all hosts, and Prometheus will be configured to scrape them directly.
For hosts that cannot send metrics via multicast (e.g. because they are
on a different subnet), *collectd* needs to listen on the all-hosts
unicast address.
Although configuration policy is not yet available for Prometheus
itself, the `collectd.yml` playbook also uses the *prometheus* host
group. Specifically, hosts in this group are configured to receive
collectd data from other hosts and expose those data through the
`write_prometheus` plugin.
