r/haproxy: Enable Prometheus metrics

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`.

roles/haproxy/templates/stats.cfg.j2

@@ -0,0 +1,10 @@
+{% if haproxy_enable_stats %}
+frontend stats
+    bind *:{{ haproxy_stats_port }}
+    http-request use-service prometheus-exporter if { path /metrics }
+    stats enable
+    stats uri /stats
+    stats refresh 10s
+{% else %}
+# HAProxy stats frontend is disabled.
+{% endif %}