Adding PCIE AER counters from class/net to ProcFs

[Diego Asturias]

I would like to add support for reading the PCIE AER counters to the ProcFs repo. I want to extend the node exporter as well to also export these metrics.

These counters are useful for monitoring PCIE errors that can disruptive. For more information about the specific please visit: 
https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-bus-pci-devices-aer_stats

I have a PR for both of these changes:

https://github.com/prometheus/procfs/pull/686

https://github.com/prometheus/node_exporter/pull/3201

The node exporter change is still a draft since I wanted to merge the procfs changes in first. 

Thanks,

Diego