[SPARK-8344] Add message processing time metric to DAGScheduler

This commit adds a new metric, `messageProcessingTime`, to the DAGScheduler metrics source. This metrics tracks the time taken to process messages in the scheduler's event processing loop, which is a helpful debugging aid for diagnosing performance issues in the scheduler (such as SPARK-4961).

In order to do this, I moved the creation of the DAGSchedulerSource metrics source into DAGScheduler itself, similar to how MasterSource is created and registered in Master.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7002 from JoshRosen/SPARK-8344 and squashes the following commits:

57f914b [Josh Rosen] Fix import ordering
7d6bb83 [Josh Rosen] Add message processing time metrics to DAGScheduler