How to list/dump dm thin pool metadata device?

See:

How to create metadata-snap for thin tools using?

I don't think LVM provides any support for metadata snapshots so you will need to drive this process through dmsetup. The kernel interface is described here:

https://github.com/torvalds/linux/blob/master/Documentation/device-mapper/thin-provisioning.txt

Don't get confused between a normal data snapshot, which takes a snapshot of a thin volume, and a metadata snapshot, which takes a snapshot of the metadata. Both of these commands that you used are for taking data snapshots (there are other reasons why those commands wouldn't work):

lvcreate -L1G -n matadata-snap -s vg0/mythinpool_tmeta
dmsetup message /dev/mapper/vg0-mythinpool 0 "create_snap metadata_snap vg0-mythinpool_tmeta"

You want to do something like:

dmsetup message /dev/mapper/vg0-mythinpool 0 "reserve_metadata_snap"

You will then be able to run thin_ls --metadata-snap on the live metadata.

Note that the kernel driver doesn't use names for thin volumes, only 32bit identifiers, so you will need to store some metadata of your own mapping user friendly names to ids if you don't want to use LVM.

Make sure you release the metadata snap as soon as you're finished with it:

dmsetup message /dev/mapper/vg0-mythinpool 0 "release_metadata_snap"

It might be helpful for you to look through this file of the dm test suite:

https://github.com/jthornber/device-mapper-test-suite/blob/master/lib/dmtest/tests/thin-provisioning/tools_tests.rb