Add support for RareTerms aggregation

[alexfrancoeur]

The Elasticsearch team is currently working on a new aggregation called the RareTerms aggregation (elastic/elasticsearch#35718, elastic/elasticsearch#20586). This will eventually deprecate / remove the ability to sort a terms aggregation by ascending count (elastic/elasticsearch#17614). Ordering this way has unbounded error and the results may not be accurate given the way a terms aggregation and shards work. The new RareTerms agg has a different algorithm that allows it to track the longtail or "rare" terms in a memory efficient manner without having to maintain a complete ordering of all terms.

If the plan is to deprecate the ability to sort a terms agg by ascending count in 7.0 and completely remove in 8.0, Kibana will need to add support for the RareTerms agg in 7.x so our users can migrate over to the more accurate algorithm.

[elasticmachine]

Pinging @elastic/kibana-app

[rrtheonlyone]

@alexfrancoeur I would like to help out with this issue. Any guidance on where I should start looking?

[polyfractal]

@rrtheonlyone The RareTerms agg hasn't merged into ES yet, although I'm hopeful it will be very soon. So work in Kibana will probably have to wait until the agg finalizes and gets merged :)

[polyfractal]

Just FYI, elastic/elasticsearch#35718 has merged. We still don't know when or if we are going to deprecate sorting count ascending on terms agg, so no rush to replace it.

RareTerms has it's own limitations and semantics so we may never be able to deprecate count ascending, despite the unbounded error aspect :( We'll see.

[Dosant]

Implemented in AggConfigs and Lens in #121500