Including node_guid in InfiniBandDevice

[di3go-sona]

Hi, at my org we are using the infiniband collector in node_exporter .

For our case it's extremely useful to also export an unique identifier like node_guid that can be read from /sys/class/infiniband/<device>/node_guid

The exporter code heavily relies on the InfiniBandClass contained here, and that identifier is not collected

https://github.com/prometheus/procfs/blob/a49c6d20a05fcee86662df0e79e5e8c59aa6197a/sysfs/class_infiniband.go#L121-L129

Is there any design reason for this choic,e or can I just contribute a PR for that ?

[discordianfish]

Sounds reasonable, so a PR would be welcome.