Description of problem: IOW a DBus API for the (equivalent of the) thin_metadata_size tool. Additional info: Required for the related libblockdev-lvm-dbus functionality as implemented in https://github.com/rhinstaller/libblockdev/pull/168 (right now using the tool even in the DBus version of the plugin).
In lvmthin(7) we just tell people to use a 1GB thin metadata LV, and say that this will always be big enough.