### METADATA CLEAR

This subcommand MUST remove all metadata, equivalently to using `METADATA SET`
on all currently-set keys with an empty value. The format of `METADATA CLEAR`
MUST be as follows:

`METADATA <Target> CLEAR`

The server MUST respond with one `RPL_KEYVALUE` event per cleared key and one
`RPL_METADATAEND` event.

Servers MAY omit certain metadata, which is considered not visible for the
requesting user.

It is an error for users to use this subcommand on targets for which they lack
authorization from the server. Servers MAY reject this subcommand for channels,
using `ERR_KEYNOPERMISSION` with an asterisk (`*`) in the `<Key>` field.