Filesystem metric to report on encrypted mounts

Feature request

I´d like to have a metric about filesystems that reports whether the filesystem is encrypted or mounted on top of an encrypted block device. I would expect special devices to be excluded from this (ie: loop devices).

For example, for the following lsblk output:

$ lsblk -e 7
NAME                        MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINTS
nvme0n1                     259:0    0 953.9G  0 disk
├─nvme0n1p1                 259:1    0   512M  0 part  /boot/efi
├─nvme0n1p2                 259:2    0   732M  0 part  /boot
└─nvme0n1p3                 259:3    0 952.7G  0 part
  └─nvme0n1p3_crypt         253:0    0 952.6G  0 crypt
    ├─vgubuntu-root         253:1    0 930.4G  0 lvm   /
    └─vgubuntu-swap_1       253:2    0   976M  0 lvm   [SWAP]

I would expect / to be marked as encrypted, but neither /boot nor /boot/efi.

For filesystems like ZFS (or future Btrfs) with native encryption support this flag should also mark those mounts as encrypted when detected.

I would suggest introducing a new gauge metric node_filesystem_encrypted with the same tags as other node_filesystem_* metrics.

Start by locating the existing node_filesystem_* metrics and the filesystem collector, then compare its behavior with the issue's lsblk -e 7 example. The finished work should expose node_filesystem_encrypted with matching tags, exclude loop devices, mark mounts backed by encrypted devices, and account for native filesystem encryption where detectable.