Add node_disk_info metric

[discordianfish]

As discussed in #304, we should add a node_disk_info metric that includes uuid and possibly other disk identifiers.
Parsing should go in procfs.

[discordianfish]

Related: #2207

[ventifus]

I've been looking into adding this. One step in that direction is prometheus/procfs/pull/412. I'll get it rebased so it's ready to merge.

[jcpunk]

I'd love to see this feature as well.

[discordianfish]

This actually has been implemented for a while, so closing this issue.