openstack instance metadata
Just browsing through the available options, it looks like the nova meta actions may be available as openstack server set:

$ openstack help server set
usage: openstack server set [-h] [--name &lt;new-name&gt;] [--root-password]
                            [--property &lt;key=value&gt;] [--state &lt;state&gt;]
                            &lt;server&gt;

Set server properties

positional arguments:
  &lt;server&gt;              Server (name or ID)

optional arguments:
  -h, --help            show this help message and exit
  --name &lt;new-name&gt;     New server name
  --root-password       Set new root password (interactive only)
  --property &lt;key=value&gt;
                        Property to add/change for this server (repeat option
                        to set multiple properties)
  --state &lt;state&gt;       New server state (valid value: active, error)


E.g., if I run:

openstack server set myserver --property foo=bar


And then run:

openstack server show myserver


I see:

[...]
| properties                           | foo='bar'                                                |
[...]