@Override
    public String getCode() {
        return "http://jabber.org/protocol/admin#get-online-users-num";
    }
Command that allows to retrieve the number of registered users who are online at
any one moment. By "online user" is meant any user or account that currently has
an IM session that may or may not be available.

@author Gaston Dombiak
@see <a href="https://xmpp.org/extensions/xep-0133.html#get-online-users-num">XEP-0133 Service Administration: Get Number of Online Users</a>
xmppserver/src/main/java/org/jivesoftware/openfire/commands/admin/GetNumberOnlineUsers.java