use gauge for MR counters

based on the discussion  https://issues.cask.co/browse/CDAP-798, we decided to keep the map reduce metrics consistent with how its reported ( as gauges). 

the PR https://github.com/caskdata/cdap/pull/726 was merged by the release manager, before i changed. so sending this change as new PR. 
yes, its used by the increment-type cdap metrics.