[RFE] add GPU support to dstat

[natoscott]

Starting to get requests for this, not too surprisingly. :)

It would be great to make GPUs a first class entity in dstat, just like CPUs, disks, network interfaces and so on, i.e. such that values for each individual card can be reported on, filtered, etc.

Here's an example simple plugin that was not merged in the old dstat that could help as a starting point for NVidia GPUs: https://blog.datumbox.com/getting-the-gpu-usage-of-nvidia-cards-with-the-linux-dstat-tool/

We also need to support the AMD GPU metrics in PCP already, and ease the way for other GPU vendors too.