[Discover] Decouple Field Statistics tab from Discover fields sidebar by refactoring how availableFields are used

[jughosta]

Original comment #135678 (comment):

I think one of the next steps could be, create a hook in Discover that could fetch the existing fields, and use it in field statistics table, remove the logic of the observable that we populate in the sidebar to use in the Field Statistic tab. feels this would be a good start for the field statistics work later on (that we should discuss separately)

Update:
Currently we use a BehaviorSubject availableFields$ that's initialized in use_saved_search.ts, updated in discover_sidebar_responsive, and subscribed to changes in field_stats_table.tsx.

Initialization:

kibana/src/plugins/discover/public/application/main/hooks/use_saved_search.ts

Line 132 in 39d1934

const availableFields$: AvailableFields$ = useBehaviorSubject(initialState) as AvailableFields$;

Updated with the actual list:

kibana/src/plugins/discover/public/application/main/components/sidebar/discover_sidebar_responsive.tsx

Line 192 in fcdcc2d

availableFields$.next({

Consumed in FieldStatisticsTable:

kibana/src/plugins/discover/public/application/main/components/field_stats_table/field_stats_table.tsx

Line 138 in 7b36f54

const fields = availableFields$?.subscribe(() => {

This should be refactored to use useExistingFieldsFetcher in FieldStatisticsTable got get the available fields like it is done in #144412. The code in use_saved_search.ts can be removed discover_sidebar_responsive

https://github.com/elastic/kibana/blob/3e8d652dfbf46bfe647e5bfab56e23ac4ffe0ee4/src/plugins/discover/public/application/main/components/sidebar/discover_sidebar_responsive.tsx#LL188C56-L188C80

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)