Spark Integration: Collect System Metrics from Spark Job

Build running here: https://builds.cask.co/browse/CDAP-DUT176-5

@anew : Please review. Thanks. 
Oh. this is from sink? Since this class is public, I have no idea who is using it. Maybe it's better to keep it package private, or even move into inner class of Sink so that it's clear that this hack is needed because we don't control the creation of the Sink object?