Add parsing of Device Mapper Slave Devices #211
Description
Add functions for parsing /sys/devices/virtual/block/dm-/slaves/ to determine state of Device Mapper Slave Devices.
This is earlier implemented as part of text-collector prometheus-community/node-exporter-textfile-collector-scripts#6 but it makes more sense to add native support integrated in the node-exporter itself via procfs library instead of having a separate Python script as text-collector.