Similarly to request in #78, please consider adding support for computing $perSecond for multiple series (hosts).

Currently, the $rateColumns computes something like:

SELECT t, arrayMap(a -> (a.1, a.2/runningDifference( t/1000 )), groupArr)
FROM (
    SELECT t, groupArray((host, v)) as groupArr 
....

Which does not produce correct results for metrics which only grows over time, as one needs to compute difference between two consequitive values v as well (not just time).

Unfortunatelly, doing runningDifference(a.2)/runningDifference(t/1000) does not work, as a.2 gets materialized to different values v between different columns (host).

I have been scratching my head for a while and I am not sure how to write such query in Clickhouse at all. I hope you know better :)