collect-metrics hook execution triggered by uniter. The uniter signals the collect-metrics hook to be executed every 5 minutes.
Merge pull request #798 from tasdomas/collect-metrics-hook

Collect metrics hook

The hook is fired every five minutes on all running units via a state server worker that updates the metrics request time field on each unit (that change is detected by the uniter watcher).
uniter: added opened-ports hook tool