[Lens] Existence fetch can take a long time

[flash1293]

In Lens, we try to determine whether fields exist in the current time range or not. To do so, 500 documents are sampled based on the current time range and filter. For each change in time range / filter, the existence data is fetched again. During the initial load of existence information, the field list is not shown:

For slow data indices (like frozen indices or searchable snapshots), this increases the time until the user can start interacting with the UI by a lot and will cause an ongoing animated loading indicator on the data panel while interacting with the editor.

One option to improve the experience would be to add a low timeout to the existence request (like 5 seconds) and fall back to showing all fields without existence information if it takes too long to fetch (something we do already if the existence fetch is not successful for other reasons):

This is related to #58330 , but we could implement it without much effort in the short term and improve behavior in these cases without solving the larger issue now.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)