Rollup aggregate: add rate / positive rate

[sandervandegeijn]

Is your feature request related to a problem? Please describe.
A lot of iot devices use counter type measurements, think energy meters, snmp counters, etc. I want to roll up this data storing only the changes (with an option for only positive rates)

Describe the solution you'd like
Add rollup aggregate for counters with both normal rate and positive rate of change

Describe alternatives you've considered
Using max and then positive rate in kibana, but not ideal

Additional context
Add any other context or screenshots about the feature request here.