Add docs for new item_count API for field data cache

[peteralfonsi]

Description

Documents the item_count metric in the field data cache API added in opensearch-project/OpenSearch#19174. Note this PR is NOT merged yet.

Also adds some changes that were missed in opensearch-project/OpenSearch#19152 (default FD size setting is now 35%, I think this was fixed in some places but not others).

Issues Resolved

Did not raise an issue for this change.

Version

3.3 and on

Frontend features

If you're submitting documentation for an OpenSearch Dashboards feature, add a video that shows how a user will interact with the UI step by step. A voiceover is optional.

Checklist

• By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and subject to the Developers Certificate of Origin.
  For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Thank you for submitting your PR. The PR states are In progress (or Draft) -> Tech review -> Doc review -> Editorial review -> Merged.

Before you submit your PR for doc review, make sure the content is technically accurate. If you need help finding a tech reviewer, tag a maintainer.

When you're ready for doc review, tag the assignee of this PR. The doc reviewer may push edits to the PR directly or leave comments and editorial suggestions for you to address (let us know in a comment if you have a preference). The doc reviewer will arrange for an editorial review.

[kolchfa-aws]

Thank you, @peteralfonsi! Doc review complete, moving on to editorial review.

[natebower]

LGTM

[natebower]

@peteralfonsi Please resolve conflicts.

[peteralfonsi]

Resolved merge conflicts. Looks like that explains why some of the FDC size setting doc changes weren't there - I must have based this branch off my own fork's main without updating. So now this PR only contains changes related to item_count which is ideal.