Wire up duration measurement of observers with monkit.
Tested by attaching a SleepObserver, starting the rangedloop in storj-up
and navigating to http://<container>:11111/mon/stats. It reports the
following statistic:
completed-observer-duration,observer=*rangedlooptest.SleepObserver,scope=storj.io/storj/satellite/metabase/rangedloop duration=10.000117
Change-Id: Ief131d34001dd5d3ba1d7be6f161986e1f66440d
