Check synchro : access only to contextcsn

Hi all, thanks for reading. I have a "supervision" account on all my ldap servers. With the plugin nagios , it check the synchro. I would like this account read only contextcsn to check synchro. And only contextcsn not the other entries. (plugin check nagios). Can someone help me to write the right ACL ?

Here what I tried but not really right :-/ # ContextCSN access to dn.subtree="dc=fr" attrs=contextCSN by dn.subtree="cn=supervision,ou=Comptes Clients,dc=fr" read by * none

Thanks.