Add text file helper for apt-get. (#680)

* Add metric for pending upgrades.
* Add metric for pending reboot required.

text_collector_examples/apt.sh

+#!/bin/bash
+#
+# Description: Expose metrics from apt updates.
+#
+# Author: Ben Kochie <superq@gmail.com>
+
+upgrades="$(/usr/bin/apt-get --just-print upgrade \
+  | /usr/bin/awk '/^Inst/ {print $5, $6}' \
+  | /usr/bin/sort \
+  | /usr/bin/uniq -c \
+  | awk '{ gsub(/\\\\/, "\\\\", $2); gsub(/\"/, "\\\"", $2);
+           gsub(/\[/, "", $3); gsub(/\]/, "", $3);
+           print "apt_upgrades_pending{origin=\"" $2 "\",arch=\"" $3 "\"} " $1}'
+)"
+
+echo '# HELP apt_upgrades_pending Apt package pending updates by origin.'
+echo '# TYPE apt_upgrades_pending gauge'
+if [[ -n "${upgrades}" ]] ; then
+  echo "${upgrades}"
+else
+  echo 'apt_upgrades_pending{origin="",arch=""} 0'
+fi
+
+echo '# HELP node_reboot_required Node reboot is required for software updates.'
+echo '# TYPE node_reboot_required gauge'
+if [[ -f '/run/reboot-required' ]] ; then
+  echo 'node_reboot_required 1'
+else
+  echo 'node_reboot_required 0'
+fi