Fix customized view aggregation latency calculation

[mgao0]

Issues

• My PR addresses the following Helix issues and references them in the PR description:
  Fixes Issues in customized view aggregation latency metric calculation #1313.

Description

• Here are some details about my PR, including screenshots of any UI changes:
  This PR contains two minor fixes to improve the customized view aggregation latency calculation.

1. Fix CustomizedViewMonitor sensor name
2. Add check for empty customized state value because it indicates a deletion and skip latency calculation for it
3. Reduce a logging level to info from warn
4. Add check for null value

Tests

• The following tests are written for this issue:

TestCustomizedViewStage -> testLatencyCalculationWithEmptyTimestamp

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

[INFO]
[ERROR] Failures:
[ERROR] TestZkConnectionLost.testLostZkConnection:162->restartZkServer:231 » ThreadTimeout
[ERROR] TestExpandCluster.testClusterExpansion:61 expected: but was:
[ERROR] TestRoutingTableProviderPeriodicRefresh.testPeriodicRefresh:211 expected:<4> but was:<3>
[ERROR] TestJobFailureDependence.testWorkflowFailureJobThreshold » ThreadTimeout Metho...
[INFO]
[ERROR] Tests run: 1174, Failures: 4, Errors: 0, Skipped: 3
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 01:26 h
[INFO] Finished at: 2020-08-25T16:05:23-07:00

Documentation (Optional)

• In case of new functionality, my PR adds documentation in the following wiki page:

(Link the GitHub wiki you added)

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)

[mgao0]

This PR is ready to be merged.
Final commit message: Fix customized view aggregation latency calculation.