Add node_filesystem_mount_info metric

[migeyel]

Mount info is now fetched from /proc/1/mountinfo (falling back to /proc/self/mountinfo), which contains strictly more information than /proc/1/mounts. This closes #1384 and introduces a workaround to #885 by letting the user join on major and minor fields.

[discordianfish]

@SuperQ I think this makes sense. I'm a bit confused why the e2e test doesn't fail due to the new metric though?

[gitperr]

I took a look at the pipeline and I think the output coming from end to end test script is not being used for some comparison. Maybe check metrics script should have been used after end to end test script ran?

[anarcat]

@migeyel could you expand here (or perhaps, better, in #600), how this can be used to "letting the user join on major and minor fields". I'm particularly interested in filtering out bind mounts from alerts here.