Add an observer to monitor ranged segment loop progress.
Tested by running the segment loop in storj-up and navigating to
http://<container>:11111/mon/stats and there is the entry:
rangedloop-live,scope=storj.io/storj/satellite/metabase/rangedloop numSegments=364523630000.000000
part of https://github.com/storj/storj/issues/5223
Change-Id: If3d2774d2f17f51eac86f47c6dda1fb8ad696dfe
