collector/diskstats: add device-mapper backing device info metric

[Maharacha]

We have this use case where we need to see what device is backing a device mapper.

Example:

node_disk_device_mapper_backing_device_info{instance="foo-bar",device="dm-0",backing_device="nvme0n1"} 1
node_disk_device_mapper_backing_device_info{instance="foo-bar",device="dm-1",backing_device="nvme0n1"} 1
node_disk_device_mapper_backing_device_info{instance="foo-bar",device="dm-2",backing_device="dm-0"} 1
node_disk_device_mapper_backing_device_info{instance="foo-bar",device="dm-2",backing_device="dm-1"} 1
node_disk_device_mapper_backing_device_info{instance="foo-bar",device="dm-3",backing_device="dm-2"} 1
node_disk_device_mapper_backing_device_info{instance="foo-bar",device="dm-4",backing_device="dm-2"} 1
node_disk_device_mapper_backing_device_info{instance="foo-bar",device="dm-5",backing_device="dm-2"} 1

Together with the already existing metric node_disk_device_mapper_info, it's now possible to map the backing devices to volume groups.

[erik78se]

This is a great addition! Well needed! PV:s are critical to surface like this. +1 from me.