This collector exposes most of the useful information that can be found
in /proc/drbd. Sizes are normalised to be in bytes, as /proc/drbd uses
kibibytes.
This change adds a new collector called "nfs" that parses the contents
of /proc/net/rpc/nfs and turns it into metrics. It can be used to
inspect the number of operations per type, but also to keep an eye on an
extraneous number of retransmissions, which may indicate connectivity
issues.
I've picked the name "nfs", as most operating systems use "nfs" for the
client component and "nfsd" as the server component. If we want to add
stats for the NFS server as well, we'd better call such a collector
"nfsd".
We seem to have a small number of Linux servers here that have lines in
/proc/mdstat that cannot be parsed by the node exporter, due to them
containing attributes that are not matched by the regular expression
("super 1.2").
Extend the regular expression to skip this data, just like we do for all
of the other status lines.
