[FLINK-19009][metrics] Fixed the downtime metric issue and updated the comment

[Flyangz]

What is the purpose of the change

Currently the way to calculate the Flink system metric "downtime" is not consistent with the description in the doc, now the downtime is actually the current timestamp minus the time timestamp when the job started. This pull request fixed this bug so that the DownTimeGauge meet the document description.

Brief change log

Calculate the downtime metric by using the current system timestamp to minus the latest failing time of the ExecutionGraph except for the following situations:

1. The start time of 'RUNNING' is greater than or equal to the latest 'FAILING' time. That means 'RUNNING' is a later status or the job has never run, and therefore downtime should be 0.
2. Jobs are terminated, so downtime is -1.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no

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Automated Checks

Last check on commit fc6dc04 (Tue Aug 25 15:41:53 UTC 2020)

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[tillrohrmann]

Thanks for creating this PR @Flyangz. LGTM. Merging this PR now.