allow to derive values

[landryb]

Testing 0.4.0 on openbsd (it works fine!), i've recently ported/tested with influxdb (so far only used rrd providers) and i'm missing one thing - i can't use 'derivative' from influxdb query language (cf https://docs.influxdata.com/influxdb/v1.4/query_language/functions/#derivative), and it seems when collectd sends values to influxdb the 'derivative' information (which in rrd is stored in the DS type) is lost, only the raw value is stored in the DB. I don't know how provider-agnostic it could be made, but it would be nice to be able to derive values in the metric options (which i've found hidden in the group tab, operation only has average/sum, and consolidation only has first/last/max/min/average/sum).

Thanks for the facette revamp !

[vbatoufflet]

Hi @landryb,

I'll have to take a closer look at this in order to see if we can do something provider-agnostic here. Like that, it won't be that easy.

[sstallion]

Somewhat related, but I have some older SNMP targets that report memory usage in terms of free and total. I'd like to be able to specify a derived used metric that is the difference of the total and free metrics.