Log correct hydrant count

[loquisgon]

Description

This PR fixes a logging bug in the BatchAppendrator class when hydrants are persisted. The bug is that the count of hydrants before this class was added was being kept in the IncrementalIndex and obtained using its getCount() method. Now the hydrant count is being kept in the new sinksMetadata that keeps tracks of Sink metadata (such as the hydrant count). The reason to change the place for keeping that count is that Sinks & Hydrants are no longer kept in memory as soon as they are persisted.

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This PR has:

• [ X] 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.
• 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.
• [X ] been tested in a test Druid cluster.