Small cleanups of the observer stats init code:
1. Use sync.Once for race free addition to the monitoring chain
(purely defensive)
2. Set the observer durations before adding to the monitoring chain on
first use.
3. observerDurations slice does not need to be initialized to non-nil
Change-Id: I9ae8ec96debc7d52c4ee5d22762a89f21bb2e38c
Support interruption of the ranged segment loop through context.
Part of https://github.com/storj/storj/issues/5223
Change-Id: Iae0260e250f8ea33affed95c6592a1f42df384eb
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
