Comment 13 for bug 1752091

I can confirm the problem on my system:

gvfsd-metadata[PID]: g_udev_device_has_property: assertion 'G_UDEV_IS_DEVICE (device)' failed
(hundrets of entries ...)

As a temporary workaround I killed (only) the gvfsd-metadata process (will auto-respawan, unless changed in systemd) and erased the local gfvs-metadata directory (will be recreated with a file in it):

$ sudo pkill gvfsd-metadata
$ rm -rf ~/.local/share/gvfs-metadata

After that the logs disappeared from my system.

Removing removing its binary /usr/libexec/gvfsd-metadata however is not a good practice, since it may lock up your system.

As a better alternative I tried to change /usr/share/dbus-1/session.conf like this (right before the </busconfig> directive/tag):

<!-- Added by TRON-DELTA.ORG for Xubuntu SE 2018 (3 lines) -->
  <policy context="default">
    <deny send_destination="org.gtk.vfs.Metadata"/>
  </policy>

and changed /usr/lib/gvfs/gvfsd-metadata via:
$ sudo chmod 000 /usr/lib/gvfs/gvfsd-metadata

Unfortunately though this led to further problems and a lock-up, not allowing to open further applications like the Thunar file explorer:

systemd[PID]: gvfs-metadata.service: Failed to execute command: Permission denied
systemd[PID]: gvfs-metadata.service: Failed at step EXEC spawning /usr/lib/gvfs/gvfsd-metadata: Permission denied

Maybe just the D-Bus rule would have been enough?