Cedric,
This list is intended for developers who write MongoDB, rather than users and developers who write software *using* MongoDB. This question is more appropriate for the mongodb-users mailing list.
However, since you're already here, if I understand you correctly, you want to grant "username" the ability to read any collection on the "local" database. That role is called "read", not "readAnyDatabase". "readAnyDatabase" is a special role on the "admin" database that grants a user the ability to read any database at all, while "read" is a role on each database that allows a user to read all collections in that database.
I hope this helps.
-Andy