Add disk I/O usage (on disk directly, not process)

[GuillaumeGomez]

On windows it can be done as follow (from @retep998):

You can query disk performance info for the entire disk and not process specific by using FSCTL_FILESYSTEM_GET_STATISTICS
or FSCTL_FILESYSTEM_GET_STATISTICS_EX if you're on windows 10

On linux, it can be retrieved from /proc/diskstats.

[WilliamVenner]

#683

[ClementTsang]

Is there still interest in this? I'm currently doing some work on implementing it for Linux/macOS/Windows https://github.com/ClementTsang/bottom anyways, so I can port over some of that code if you want.

My only question is whether we also need implementations/dummy implementations for the other supported OSes.

[GuillaumeGomez]

I'm definitely interested. From what you wrote, you don't plan to add FreeBSD but it's fine, I can add the missing parts.