Updating SQL Metrics Documentation

[ashwintumma23]

Description

Fixed the bug ...

• SQL Metrics offer engine as a dimension as well, looks like the documentation update was missed for this dimension.
• Code reference: https://github.com/apache/druid/blob/master/sql/src/main/java/org/apache/druid/sql/SqlExecutionReporter.java#L98
• https://druid.apache.org/docs/latest/operations/metrics/#broker (Broker Metrics) section documents it correctly

Release note

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Key changed/added classes in this PR

• docs/operations/metrics.md

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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.