Add Slf4jSink to Spark Metrics using Coda Hale's SlfjReporter.
This sends metrics to log4j, allowing spark users to reuse log4j pipeline for metrics collection.

Reviewed existing unit tests and didn't see any sink-related tests. Please advise on if tests should be added.

Author: Judy <judynash@microsoft.com>
Author: judynash <judynash@microsoft.com>

Closes #4644 from judynash/master and squashes the following commits:

57ef214 [judynash] doc clarification and indent fixes
a751a66 [Judy] Spark-5708: Add Slf4jSink to Spark Metrics