HAProxy can export stats in Prometheus format, but this requires
special configuration of a dedicated front-end. To support this, the
_haproxy_ Ansible role now has a pair of variables,
`haproxy_enable_stats` and `haproxy_stats_port`, which control whether
or not the stats front-end is enabled, and if so, what port it listens
on. Note that on Fedora with the default SELinux policy, the port must
be labelled either `http_port_t` or `http_cache_port_t`.
Dustin