[Feature] Rollup: add (non-negative) derivative

[sandervandegeijn]

Is your feature request related to a problem? Please describe.
Rollups support several types: avg, sum, etc. But there are counters, i.e. energy counters and snmp-traffic counters, that are represented by an increasing number. I'd like to store the delta's for these counters during the roll-up phase

Describe the solution you'd like
InfluxDB (and other time series databases) support something like a (non-negative-)derivative function to do this. It'll calculate the difference between the buckets and if the counter resets it'll ignore the negative value (and implement another function that doesn't do that btw)

Describe alternatives you've considered
Rollup with max and then using the dashboards visualisations function. But this only applies to dashboards and there are more use cases that could be served through the API.

Additional context
N/A

[ashking94]

@ict-one-nl - Can you elaborate more on the feature request?

[sandervandegeijn]

Sure, what would you need? It's all about counter (some reset after power loss / end of day, some don't). You want to store the differences so you can easily calculate how much data was transferred or energy was used.