Load exact hit count async

[timroes]

The new format of hits.total will by default only calculate estimated counts, which can improve querying performance significantly.

Since we don't need an exact count in Discover, I would suggest by default we're actually showing the estimated count, with some info icon behind it. If you click this, you would get a popover (similar to the KQL one in the query bar), explaining you, that we only show estimated counts, due to performance, and give the user a switch, which which they could opt into exact counts, in which case we would request the exact amount and show it (that switch could be saved with the discover search).

Update: In some discussion with Vijay we decided that we might just do the query to count the documents always (or have an advanced setting to configure it), just also as a separate request, and just show the "estimated hits" that will be returned with the document query, until the count query is done and we then just switch the estimate hits by the real hit count.

Part of #69134
Related #26356
Blocked by #55974

[elasticmachine]

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

[timroes]

Liza has done some measures of different queries (see this comment) which gives us a good understanding of HOW FAST actually we'll get by not using the document query anymore to load the exact hit time.

We can improve the performance potentially 10 times (look at the "LATEST w/totals, w/sort" vs "LATEST wo/totals, w/sort" column) for larger clusters.

[timroes]

Closing in favor of #69134