systemd: Clarify private flag description (#1587)

This requires root, so it shouldn't be used.

This closes #1246

Signed-off-by: Johannes 'fish' Ziemke <github@freigeist.org>

collector/systemd_linux.go

@@ -41,7 +41,7 @@ const (
 var (
 	unitWhitelist          = kingpin.Flag("collector.systemd.unit-whitelist", "Regexp of systemd units to whitelist. Units must both match whitelist and not match blacklist to be included.").Default(".+").String()
 	unitBlacklist          = kingpin.Flag("collector.systemd.unit-blacklist", "Regexp of systemd units to blacklist. Units must both match whitelist and not match blacklist to be included.").Default(".+\\.(automount|device|mount|scope|slice)").String()
-	systemdPrivate         = kingpin.Flag("collector.systemd.private", "Establish a private, direct connection to systemd without dbus.").Bool()
+	systemdPrivate         = kingpin.Flag("collector.systemd.private", "Establish a private, direct connection to systemd without dbus (Strongly discouraged since it requires root. For testing purposes only).").Hidden().Bool()
 	enableTaskMetrics      = kingpin.Flag("collector.systemd.enable-task-metrics", "Enables service unit tasks metrics unit_tasks_current and unit_tasks_max").Bool()
 	enableRestartsMetrics  = kingpin.Flag("collector.systemd.enable-restarts-metrics", "Enables service unit metric service_restart_total").Bool()
 	enableStartTimeMetrics = kingpin.Flag("collector.systemd.enable-start-time-metrics", "Enables service unit metric unit_start_time_seconds").Bool()