collectd: Add DaemonSet for collectd

Since all the nodes in the cluster run Fedora CoreOS now, we can
deploy collectd as a container, managed by a DaemonSet.

Note that while _collectd_ has to run as _root_ in order to collect
a lot of metrics, it should not run with all privileges.  It does need
to run as a "super-privileged container" (`spc_t` SELinux domain), but
it does _not_ need most kernel capabilities.

collectd/collectd.d/df.conf

@@ -0,0 +1,10 @@
+LoadPlugin df
+
+<Plugin df>
+    ReportByDevice true
+
+    FSType autofs
+    FSType overlay
+    FSType efivarfs
+    IgnoreSelected true
+</Plugin>

collectd/collectd.d/log.conf

@@ -0,0 +1,8 @@
+LoadPlugin logfile
+
+<Plugin logfile>
+	LogLevel info
+	File stderr
+	Timestamp false
+	PrintSeverity true
+</Plugin>

collectd/collectd.d/plugins.conf

@@ -0,0 +1,9 @@
+LoadPlugin chrony
+LoadPlugin cpufreq
+LoadPlugin disk
+LoadPlugin entropy
+LoadPlugin processes
+LoadPlugin swap
+LoadPlugin tcpconns
+LoadPlugin thermal
+LoadPlugin uptime

collectd/collectd.d/prometheus.conf

@@ -0,0 +1,5 @@
+LoadPlugin write_prometheus
+
+<Plugin write_prometheus>
+    Port 9103
+</Plugin>