prometheus: collectd: Listen on unicast socket

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.

group_vars/prometheus.yml

@@ -1,4 +1,5 @@
 collectd_network_servers: []
 collectd_network_listen:
+- '::'
 - ff18::efc0:4a42
 collectd_prometheus_host: '::1'