r/collectd: Ignore overlay filesystems

By default, the *df* pluggin for collectd, which monitors filesystem
usage, collects data about all mounted filesystems.  It can be
configured to ignore some filesystems, either by mount point, device, or
filesystem type.  We will uses this capability to avoid collecting data
about OverlayFS mounts, because by definition, they do not represent a
real filesystem, but one or more other mounted filesystems.  Collecting
data about these just creates useless metrics, especially on machines
that run containers.

roles/collectd/tasks/main.yml

@@ -50,6 +50,16 @@
   - collectd-config
   - collectd-typesdb
 
+- name: ensure collectd df plugin is configured
+  template:
+    src: collectd-df.conf.j2
+    dest: /etc/collectd.d/df.conf
+    mode: 'u=rw,go=r'
+  notify: restart collectd
+  tags:
+  - collectd-config
+  - collectd-df
+
 - name: ensure collectd unixsock plugin is configured
   template:
     src: collectd-unixsock.conf.j2