[SPARK-7007] [CORE] Add a metric source for ExecutorAllocationManager

Add a metric source to expose the internal status of ExecutorAllocationManager to better monitoring the resource usage of executors when dynamic allocation is enable. Please help to review, thanks a lot.

Author: jerryshao <saisai.shao@intel.com>

Closes #5589 from jerryshao/dynamic-allocation-source and squashes the following commits:

104d155 [jerryshao] rebase and address the comments
c501a2c [jerryshao] Address the comments
d237ba5 [jerryshao] Address the comments
2c3540f [jerryshao] Add a metric source for ExecutorAllocationManager