Add latency metric support for customized view aggregation

[mgao0]

Issues

• My PR addresses the following Helix issues and references them in the PR description:

Fixes #1214

Description

• Here are some details about my PR, including screenshots of any UI changes:

This PR adds support for latency metric reporting between the time when user updates customized state for a partition using CustomizedStateProvider, and the time this change shows up in aggregated customized view and written to zk. With that, we can 1. compare this latency with other pipeline latencies, so we know if this newly added feature saturate our controller or not 2. get an understanding of the pipeline performance 3. better monitor this feature and diagnose any problems in the future

• The following tests are written for this issue:

Added:

1. TestCustomizedViewMonitor
2. TestCustomizedViewStage -> testLatencyMetricReporting

Modified:

1. TestCustomizedStateUpdate
2. TestCustomizedViewAggregation

• The following is the result of the "mvn test" command on the appropriate module:

[INFO]
[INFO] Results:
[INFO]
[WARNING] Tests run: 1168, Failures: 0, Errors: 0, Skipped: 1
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 01:13 h
[INFO] Finished at: 2020-08-13T19:47:48-07:00
[INFO] ------------------------------------------------------------------------

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)

[jiajunwang]

LGTM!

[mgao0]

This PR is ready to merge, approved by @jiajunwang
Final commit message:
Add latency metric support for customized view aggregation