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
(cherry picked from commit 0afe5cb65a195d2f14e8dfcefdbec5dac023651f)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>