Kafka emitter wasn't given the correct number of threads.

[TSFenwick]

Fixes issue with kafka emitter not having the correct number of threads

Description

The kafka emitter didn't have the correct number of threads. It should be 1 thread per scheduled task. If the kafka emitter was emitting every eventType the lost event logging callable would never get called.

This change intelligently provisions the correct number of threads per scheduled task. 1 for each event type, and 1 for logging the lost events. This is a change to make this work. But in the future it would be worthwhile to make each task not be greedy and share threads so there isn't a need of a thread per task.

Release note

Fixed: KafkaEmitter will now run all scheduled callables. It now intelligently provision threads to make sure there are no wasted threads, and all callables can run.

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Key changed/added classes in this PR

• KafkaEmitter
• KafkaEmitterConfig

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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.