collectd: Install and configure collectd

The `collectd.yaml` Butane configuration fragment configures the machine
to install *collectd* and its various plugin packages directly on the
host using `rpm-ostree` (via *install-packages.service*).

collectd.yaml

@@ -0,0 +1,57 @@
+variant: fcos
+version: 1.4.0
+
+ignition:
+  config:
+    merge:
+    - local: packages.yaml
+
+storage:
+  files:
+  - path: /etc/ignition/packages.d/collectd
+    mode: 0644
+    contents:
+      inline: |
+        collectd
+        collectd-chrony
+        collectd-disk
+        collectd-sensors
+        collectd-write_prometheus
+  - path: /etc/collectd.d/df.conf
+    mode: 0644
+    contents:
+      inline: |
+        LoadPlugin df
+
+        <Plugin df>
+            FSType overlay
+            IgnoreSelected true
+        </Plugin>
+  - path: /etc/collectd.d/plugins.conf
+    mode: 0644
+    contents:
+      inline: |
+        LoadPlugin chrony
+        LoadPlugin cpufreq
+        LoadPlugin disk
+        LoadPlugin entropy
+        LoadPlugin processes
+        LoadPlugin swap
+        LoadPlugin tcpconns
+        LoadPlugin thermal
+        LoadPlugin uptime
+
+  - path: /etc/collectd.d/prometheus.conf
+    mode: 0644
+    contents:
+      inline: |
+        LoadPlugin write_prometheus
+        
+        <Plugin write_prometheus>
+            Port 9103
+        </Plugin>
+
+systemd:
+  units:
+  - name: collectd.service
+    enabled: true