CDAP-484 User and System Metrics Gauge Support

Gauge support for User and System Metrics and updated Mapreduce Metrics Writer to use gauge

https://issues.cask.co/browse/CDAP-484
Reading about this, this metric  would be gauged when the executor is not shutdown, but the queue (holding requests) is full and no new threads can be created by the ThreadExecutorFactory, is that correct ? so, does it make sense to use gauge ?, as this can be called by multiple incoming requests ? . moreover this metric name is not reference anywhere else , @chtyim  what would be the right thing to do ?