Metrics

[josiahsams]

As part of this pull request the following capabilities are added,

1. Ability to include user custom metrics into spark generated code is added.
2. A listener is provided to report the user metrics at the end of every Stage.

Usage:
Now the user generated code will have similar code included,

this.userDefined1 = (org.apache.spark.sql.catalyst.UserTaskMetric) references[0];

Operations like add, += are supported on this new object as,

userDefined1.add(100);

To get the custom metrics report, include the provided listener to the SparkListeners as follows in either spark-shell or spark-submit,

--conf spark.extraListeners=org.apache.spark.sql.catalyst.UserStatsReportListener