SPARK-3028. sparkEventToJson should support SparkListenerExecutorMetrics...

...Update

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1961 from sryza/sandy-spark-3028 and squashes the following commits:

dccdff5 [Sandy Ryza] Fix compile error
f883ded [Sandy Ryza] SPARK-3028. sparkEventToJson should support SparkListenerExecutorMetricsUpdate