TTL Node Interface Methods

[Marcosrico]

Issues

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

#2237

Description

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

Interface methods for creating and renewing TTL methods. Added extra instance variables in MetaClient.Stat for future use cases. Please check design document for reasoning.

Tests

• The following tests are written for this issue:

(List the names of added unit/integration tests)

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

(If CI test fails due to known issue, please specify the issue and test PR locally. Then copy & paste the result of "mvn test" to here.)

Failed tests are from known issues. Below are the results from local testing:

[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 104.186 s - in org.apache.helix.monitoring.mbeans.TestClusterAggregateMetrics
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 163.445 s - in org.apache.helix.monitoring.TestClusterStatusMonitorLifecycle
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO] 

Code Quality

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

[qqu0127]

Overall LGTM, one nit on documentation

[mgao0]

lgtm

[xyuanlu]

Thanks for the change. Please rerun failed test in CI.

[Marcosrico]

This PR is ready to merge, approved by @mgao0 @qqu0127 @xyuanlu
Commit message: TTL Node Interface Methods