Add orphans apt packages metric

roles/prometheus-node-exporter/files/apt.sh

@@ -19,6 +19,13 @@ autoremove="$(/usr/bin/apt-get --just-print autoremove \
   | /usr/bin/awk '/^Remv/{a++}END{printf "apt_autoremove_pending %d", a}'
 )"
 
+orphans="$(comm -23 \
+  <(dpkg-query -W -f '${db:Status-Abbrev}\t${Package}\n' \
+    | grep '^.[^nc]' | cut -f2 | sort) \
+  <(apt-cache dumpavail | sed -rn 's/^Package: (.*)/\1/p' | sort -u) \
+  | awk 'END{printf "apt_orphans %d", NR}'
+)"
+
 echo '# HELP apt_upgrades_pending Apt package pending updates by origin.'
 echo '# TYPE apt_upgrades_pending gauge'
 if [[ -n "${upgrades}" ]] ; then
@@ -31,6 +38,10 @@ echo '# HELP apt_autoremove_pending Apt package pending autoremove.'
 echo '# TYPE apt_autoremove_pending gauge'
 echo "${autoremove}"
 
+echo '# HELP apt_orphans Orphan apt package.'
+echo '# TYPE apt_orphans gauge'
+echo "${orphans}"
+
 echo '# HELP node_reboot_required Node reboot is required for software updates.'
 echo '# TYPE node_reboot_required gauge'
 if [[ -f '/run/reboot-required' ]] ; then