Hi Gevorg,
Other buckets work by doing two requests against Elasticsearch. One which will actually retrieve the terms aggregation and it's values as if you would not have specified other terms. After that has returned, we are doing a second request, which is basically a request using the same metric aggregations, but is using a filter, to filter out all of the terms that has been returned by the first request in the terms aggregation.
In case you've got multiple nested terms aggregations, we will include a filter aggregation for every level in the second request.
What metric you actually choose doesn't matter. It works the same with Count as with any other metric, since we do the second request with a different filters bucket aggregation, but can use the very same metrics aggregations.
With Kibana 6.4, we are replacing the existing spy panel, by a new solution, which will also be able to show you both requests (the data request and the other bucket request), so you can gain better insights into that.
Hope that answer contained all the information you where looking for. If not please feel free to get back with any questions.
Cheers,
Tim