Update Documentation on meregeBuffer/pendingRequests for Real-time nodes

[hardikbajaj]

Fixes #XXXX.
#15025 adds mergeBuffer/pendingRequests metric in QueryCountStatsMonitor. Since Real-time nodes also uses same merge buffers for queries and have QueryCountStatsMonitor , updating the documentation to include this metric.

Description

Fixed the bug ...

Renamed the class ...

Added a forbidden-apis entry ...

Release note

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Key changed/added classes in this PR

• MyFoo
• OurBar
• TheirBaz

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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.

[LakshSingla]

Thanks for the patch!