Fix bug in auto compaction preserveExistingMetrics feature

[maytasm]

Fix bug in auto compaction preserveExistingMetrics feature

Description

This PR contains the following changes:

• Auto compaction should not set preserveExistingMetrics if metricsSpec is not explicitly set in auto compaction config. This falls back to current behavior before metricsSpec was added to auto compaction and Compaction task would be able to automatically discover and generate the correct combining metricsSpec without needing to use preserveExistingMetrics function
• When preserveExistingMetrics is use, some Aggregators may return different Type in it's Combine method. For example, the FloatSum takes in two Float but returns a Double in its Combine method. This PR handle this case by converting the returned Object to a Number object then calling the Number Object's method to get the correct Type value.

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.
• 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.