I/O Density calculation

[jgasher]

Is your feature request related to a problem? Please describe. I've not been able to make an I/O Density calculation work in Grafana using the built-in functions, but I don't think it would be hard to implement in Harvest.

Describe the solution you'd like Simply divide the IOPS by the space used. Ideally we could get IOPS divided by storage used before and after efficiencies and also the volume size. I think this could be handled in a similar way to calculating the FCP port utilization is calculated in the FCP collector plugin.

Describe alternatives you've considered I've tried Grafana functions to calculate values and I can get it to work for a particular volume, but that won't work with templates and being able to select volumes from the list. The dashboards only work if the volume is literally specified in the query string.