Add support for Linux SAS classes

[scottlaird]

Linux exports ~5 useful metrics per SAS PHY, which can be useful for debugging SAS performance problems and errors. I'm planning on adding support for this to node_exporter (see prometheus/node_exporter#2386), but I need to extract information from /sys first before I can do this.

The useful metrics are all in /sys/class/sas_phy, but we need data from /sys/class/sas_end_device, /sys/class/sas_port, and /sys/class/sas_expander in order to make sense of the PHY data and know what's connected to a specific PHY.

[hasechris]

@scottlaird
This issue is quite old. Can i help here in any way to speed things up? I would really love to have this integration in node_exporter.

[scottlaird]

Ah, I think I can close this one, but the node exporter issue is still open. I have the code for that, but I never put a PR together. I'll work on it soon.