Add filter flag to mountstats

[SuperQ]

node_exporter version: output of node_exporter --version

0.18.1

node_exporter command line flags

--collector.mountstats

Feature proposal

When mounting the same export multiple times the metrics are duplicated for each virtual mountpoint. There doesn't seem to be an easy method to de-duplicate this by understanding the underlying NFS client connection.

A simple workaround would be to add a manual ignore filter based on the export label. This would allow for exporter-side filtering of metrics. It may also be useful in other situations where only some NFS mountstats metrics are desired.

[dipack95]

@SuperQ This might not be strictly related to this issue, but we notice that a lot of the times, mounts originating from the same machine (with different options), are dropped and not reported by the mountstats collector.

After reading through the spec for storing information in /proc/self/mounststats, which is where the mount information is parsed from, it seems that the stats reported there are for each unique NFS connection to the remote machine. A unique NFS connection is identified by the remote machine's address, and the set of options used while opening the connection. If multiple mounts with the same options are created on the local machine, their stats are identical in the mountstats file.

In the current implementation of the NFS metrics collector, it does not understand this, and simply checks for the 'uniqueness' of a NFS mount by comparing the remote mount, network protocol, and as of this PR, the remote machines IP address.

Do you have any ideas on how to improve this?

[SuperQ]

#1400 is a good start. If we find additional fields that we need to add labels for to provide enough uniqueness, we can add them.

[dipack95]

Well, even after adding a mountaddr field that represents the remote machine's IP address, we still see mounts being ignored.

One way to tackle this issue, that I can think of, would be expand the nfsDeviceIdentifier struct to also contain the options with which a particular mount was created. This is will ensure that legitimate mounts from the same machine are not ignored, if they don't have the same options.

The downside is that the metric names will contain a long list of values (mount options), which help uniquely identify each NFS connection + mount.