Spark Integration: Custom User Metrics

Added support for emitting custom user metrics from Spark Programs.

Build running here: https://builds.cask.co/browse/CDAP-RBT10-1


@anew : Please review. Thanks.
No I just added the support in my Spark Example to emit custom metrics.

But I cannot verify querying our Metrics system in unit testing to check for the value. 

Our current unit testing system for Metrics emits system metrics to RunTimeStats so we can query it and assert. But the user metrics are not available there. So, I opened a jira issue to ass support for that.