[bugfix] Run cold schema refresh thread periodically

[findingrish]

Issue

Parent issue: #14989
Fixes a bug in #16676.
Schema from cold segments wasn't getting reflected in the datasource schema.
The problem turned out to be in the refresh thread, it ran only once instead of running periodically.

This PR also adds some metrics to monitor cold schema refresh process.

Testing

1. Added unit test
2. Verified in a Druid cluster that,
   • Completely cold datasource is queryable. Refer to cold datasource in the attached screen shot with 1 segment.

• Datasource schema for partially cold datasource includes columns from cold segments. Refer to partially_cold datasource in the attached screen shot with 2 segments. The hot segment has columns c3, c4, c5 and cold segment has columns c1, c2, c3.

• Schema for new cold segments created after enabling the CentralizedDatasourceSchema feature is included in datasource schema.
• Entirely cold datasources which existed before enabling CentralizedDatasourceSchema will not be queryable until at least 1 segment is loaded after enabling the feature. The datasource remains queryable even if the segment is unloaded.

Release Notes

Added followings metrics to monitor cold schema refresh process.

metadatacache/cold/segment/count -> Number of cold segment per datasource. 
metadatacache/cold/refresh/count -> Number of cold segments with cached schema per datasource. 
metadatacache/cold/process/time -> Time taken in milliseconds to process cold segments. 

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.

[cryptoe]

Nit: Use a prefix for metadatacache/cold/segment/

[kfaraz]

"cold segment" is not a term that exists anywhere outside of CoordinatorSegmentMetadataCache.
It would be better to use a term that is more easily relatable.

Also, the metadatacache metrics seems all over the place with their prefixes. It is unavoidable for metrics across multiple features to not have a consistent naming scheme. But metrics within the same feature should be consistent. Since this feature is still nascent, I would advise we revisit all the metric names for this feature and categorize them nicely with proper prefixes. (not necessarily in this PR)

cc: @cryptoe

[findingrish]

It would be better to use a term that is more easily relatable.

I don't find much references for such segments, do you have any suggestions?

Since this feature is still nascent, I would advise we revisit all the metric names for this feature and categorize them nicely with proper prefixes.

Are you suggesting to use a different prefix than metadatacache? How about cds?

[cryptoe]

Yes lets change the wording from cold to deepstorageonly.