Use base units and add unit to metric name

[discordianfish]

Hi,

first of all, thanks for the exporter! It works great in my home network.
Ideally though it would follow Prometheus' best practices of using base units: https://prometheus.io/docs/practices/naming/#base-units

That would mean changing prosafe_link_speed to prosafe_link_speed_bytes, similarly to the node-exporter's node_network_speed_bytes.

I can submit a PR to change this if this is acceptable.

[dalance]

Thanks. I welcome your PR.