[Feature Request] Add item count metric to field data cache stats API

[peteralfonsi]

Is your feature request related to a problem? Please describe

While debugging for #13862 @sgup432 mentioned it would have been convenient to have an item count stat for the field data cache. Today we only have evictions and memory size (total + per field).

Describe the solution you'd like

We should add an item_count metric to the existing _nodes/stats/indices/fielddata API. It can optionally be aggregated by field, like memory size is.

Related component

Other

Describe alternatives you've considered

No response

Additional context

No response

[sgup432]

Thanks for opening this issue, would very useful to have an item_count for FDC