[Feature]: Should graph the 'c' UoM as derivative

[OdyX]

Describe the feature request

According to https://icinga.com/docs/icinga-2/latest/doc/05-service-monitoring/#performance-data-metrics:

The UoM “c” represents a continuous counter (e.g. interface traffic counters).

Yet these (otherwise excellent) graphs display "ever-increasing" metrics, where they should graph these counters as a derivative over time.

Of course it's possible to implement caching on the plugin side (and then calculate occurences/minute from the last cached entry), but that should really be handled at the collecting side (same as munin).

[martialblog]

Hi, thanks for the feedback.

Yeah, counters are tricky. Right now I don't have a clear idea on how we make the graphs for counters more useful.
A Prometheus-like "rate()" would be nice, however we would need to implement ourselves, since not all backends prove these functions. But this is a lot of complexity we would introduce in the module.

[martialblog]

While working on this #52 I figured that maybe we can use a similar logic for the counters. We have all the datapoints and the check interval, that might be enough to calculate a meaningful derivative.

Where an when to calculate that is still a question.