Spark Integration: Collect System Metrics from Spark Job

Build running here: https://builds.cask.co/browse/CDAP-DUT176-4

@anew : Please review. Thanks. 
good. can you also change, https://github.com/caskdata/cdap/blob/develop/cdap-gateway/src/test/java/co/cask/cdap/gateway/handlers/metrics/MetricsQueryTest.java#L140