+II.6 Update client "metadata"
+
+ Command: METADATA
+ Parameters: <target> <key> <value>
+ Used by: servers only
+
+The METADATA command is used on server-links to update "metadata" information
+of clients, like the hostname, the info text ("real name"), or the user name.
+
+The server updates its client database according to the received <key> and
+<value> parameters, and passes the METADATA command on to all the other
+servers in the network that support this command (see section II.1 "Register
+new server link", <serverflag> "M"), even if it doesn't support the given
+<key> itself: unknown <key> names are ignored silently!
+
+The following <key> names are defined:
+
+ - "accountname": the account name of a client (can't be empty)
+ - "certfp": the certificate fingerprint of a client (can't be empty)
+ - "cloakhost": the cloaked hostname of a client
+ - "host": the hostname of a client (can't be empty)
+ - "info": info text ("real name") of a client
+ - "user": the user name of a client (can't be empty)
+
+