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
I think the MetricsTable API (the type of the aggregatesTable) shouldn't expose method that accepts raw bytes, as it requires value to be `long`. Maybe it's good time to fix that API.