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

[JoshRosen]

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.

[JoshRosen]

/cc @kayousterhout or @markhamstra

[JoshRosen]

@aarondav, this is the metric that I mentioned.

[SparkQA]

Test build #35733 has finished for PR 7002 at commit 57f914b.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class TakeOrderedAndProject(

[kayousterhout]

LGTM!

[JoshRosen]

I'm going to merge this into master. Thanks!

[akashsethi24]

@JoshRosen@kayousterhout I Know this is old PR but I just want to know for messageProcessingTime value like mean, mean_rate are calculated and shown for each fresh batch or aggregated with the old mean and mean_rate?
Thanks