[SPARK-4595][Core] Fix MetricsServlet not work issue

`MetricsServlet` handler should be added to the web UI after initialized by `MetricsSystem`, otherwise servlet handler cannot be attached.

Author: Saisai Shao <saisai.shao@intel.com>
Author: Josh Rosen <joshrosen@databricks.com>
Author: jerryshao <saisai.shao@intel.com>

Closes #3444 from jerryshao/SPARK-4595 and squashes the following commits:

434d17e [Saisai Shao] Merge pull request #10 from JoshRosen/metrics-system-cleanup
87a2292 [Josh Rosen] Guard against misuse of MetricsSystem methods.
f779fe0 [jerryshao] Fix MetricsServlet not work issue
[SPARK-4625] [SQL] Add sort by for DSL & SimpleSqlParser

Add `sort by` support for both DSL & SqlParser.

This PR is relevant with #3386, either one merged, will cause the other rebased.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #3481 from chenghao-intel/sortby and squashes the following commits:

041004f [Cheng Hao] Add sort by for DSL & SimpleSqlParser