Log admin task

[Marco]

Hello Cyrus users,

  I have the need to log admin activity on users. With  
Cyrus-imapd-2-4-17, if I use admin to authenticate as another user,  
such as:

   imtest -u pooruser at example.com -a admin -m plain localhost

then in log I see only <pooruser at example.com> references, nothing  
about <admin>:

Oct 15 09:06:23 tst cyrus/imap[28159]: login: localhost.localdomain  
[127.0.0.1] pooruser at example.com PLAIN User logged in  
SESSIONID=<cyrus-28159-1381820778-1>
Oct 15 09:06:30 tst cyrus/imap[28159]: USAGE pooruser at example.com  
user: 0.012998 sys: 0.008998


How can I configure imap log to trace <admin> in these circumstances?

Thanks a lot
Marco