puppet: Enable prometheus monitoring of supervisord.

To be able to read the UNIX socket, this requires running
node_exporter as zulip, not as prometheus.

puppet/zulip_ops/files/supervisor/conf.d/prometheus_node_exporter.conf

@@ -1,8 +1,8 @@
 [program:prometheus_node_exporter]
-command=/usr/local/bin/node_exporter --no-collector.hwmon
+command=/usr/local/bin/node_exporter --collector.supervisord --collector.supervisord.url=unix:///var/run/supervisor.sock --no-collector.hwmon
 priority=10
 autostart=true
 autorestart=true
-user=prometheus
+user=zulip
 redirect_stderr=true
 stdout_logfile=/var/log/node_exporter.log

puppet/zulip_ops/manifests/prometheus/node.pp

@@ -22,7 +22,7 @@ class zulip_ops::prometheus::node {
   file { "${zulip::common::supervisor_conf_dir}/prometheus_node_exporter.conf":
     ensure  => file,
     require => [
-      User[prometheus],
+      User[zulip],
       Package[supervisor],
       File['/usr/local/bin/node_exporter'],
     ],