Somewhen @metadata::trusted yes@ whas the right thing to do, but nowadays
you have to use @metadata::trusted true@. What was taking me one day to
spot. The internet has still a lot reference to @metadata::trusted yes@.

I added also Requires and After to the service file, to make sure
systemd starts gvfs-metadata deamon before ours. We need this DBus
interface to be availabe to get gio working.