Add Master Throttling Metric

[dhwanilpatel]

Description of changes:
Add Metrics for Master Throttling of Pending tasks. It will publish two metric 1) Total throttled tasks at master node 2) Number of task on which data node is actively retrying. This feature is yet to be contributed to Opensource ES. I have added the check for availability of this feature in collector so building with or without this feature will pass. If this feature is not available then collector will simply return.

PR of Performance Analyzer Plugin : opendistro-for-elasticsearch/performance-analyzer#217

Tests:

Testing

1. /gradlew test Successful

SUCCESS: Executed 455 tests in 40m 43s (1 skipped)

BUILD SUCCESSFUL in 41m 8s
11 actionable tasks: 8 executed, 3 up-to-date

2. Tested in test domain by replacing JAR.

Tmp file

^master_throttling_metrics
{"current_time":1602617137529}
{"Data_RetryingPendingTasksCount":0,"Master_ThrottledPendingTasksCount":0}$

Verified metric from table

sqlite> select * from Master_ThrottledPendingTasksCount;
0.0|0.0|0.0|0.0

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[yojs]

The CI isn't passing. There are some spot bug failures. You can rerun the spotbugs as
./gradlew spotbugsMain

And then point your browser to <project dir>/build/reports/spotbugs/main.html
That will give you an idea of what the errors are.

[adityaj1107]

There are some checkstyle errors in the CI. Fix them and merge the changes in the RCA packages first since PA depends on RCA to build (for Constants etc.).