Fix messageGap emission

[AmatyaAvadhanula]

Do not emit messageGap after streaming tasks stop processing events.

Description

Modify the emission of metrics to stop emitting messageGap after a task has stopped processing events on its own or due to a stop request etc.

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Key changed/added classes in this PR

• SeekableStreamIndexTaskRunner
• FireDepartmentMetrics

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[kfaraz]

LGTM, thanks for the quick fix, @AmatyaAvadhanula !