Report upload/download speed via Prometheus?

**is your feature request related to a problem? Please describe.**
Not really.

**Describe the idea / solution you'd like**
I'd like to know how much upload/download in total or per-volume the server is doing.

**Describe any alternatives you've considered**
I looked at all the metrics and couldn't see a way. Only ways to know how many downloads or unfinished uploads are happening at any one time.

**Additional context**
None.