Decorate sampling response with system fields if specified

[zachjsh]

Description

#15276 added the ability to consume system fields from certain input sources. These
systemField values were previously however not being properly decorated in the sampling response, not allowing the user to sample ingest data whose value depends on the values of such systemFields. This pr fixes this issue.

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.