[discover] esql reduce field_caps usage

[mattkime]

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Risk Matrix

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When forming the risk matrix, consider some of the following examples and how they may potentially impact the change:

Risk	Probability	Severity	Mitigation/Notes
Multiple Spaces—unexpected behavior in non-default Kibana Space.	Low	High	Integration tests will verify that all features are still supported in non-default Kibana Space and when user switches between spaces.
Multiple nodes—Elasticsearch polling might have race conditions when multiple Kibana nodes are polling for the same tasks.	High	Low	Tasks are idempotent, so executing them multiple times will not result in logical error, but will degrade performance. To test for this case we add plenty of unit tests around this logic and document manual testing procedure.
Code should gracefully handle cases when feature X or plugin Y are disabled.	Medium	High	Unit tests will verify that any feature flag or plugin combination still results in our service operational.
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[mattkime]

This is identified in #169835 and is where my changes start but they seem to ripple outward. Someone with more experience in the discover state management could probably help me come up with a strategy.

[kertal]

I had a look at this, and we can't currently use DataViewLazy here, because this is setting the dataView in state which expects the DataView that's not so Lazy

So there's one thing in ES|QL that needs to be resolved differently. When the query is changed, the change of index pattern / data view is checked, and this can lead to loading time without any loading indication ... so we currently can't reduce field caps for ES|QL, but we could improve loading communication to the user, which happens here, when a query is submitted:

kibana/src/plugins/discover/public/application/main/services/discover_data_state_container.ts

Lines 299 to 318 in edd384a

	const fetchQuery = async (resetQuery?: boolean) => {
	const query = getAppState().query;
	const currentDataView = getSavedSearch().searchSource.getField('index');
	if (isTextBasedQuery(query)) {
	// this is where the dataView gets set to state, making it difficult to swap with dataViewLazy
	const nextDataView = await getDataViewByTextBasedQueryLang(query, currentDataView, services);
	if (nextDataView !== currentDataView) {
	setDataView(nextDataView);
	}
	}
	if (resetQuery) {
	refetch$.next('reset');
	} else {
	refetch$.next(undefined);
	}
	return refetch$;
	};