Thank you Devrim!
The "man 5 pgbouncer" is slightly outdated then,
it doesn't explain the auth_file format in detail:
auth_file
The name of the file to load user names and passwords from. The file format is the same as the PostgreSQL
pg_auth/pg_pwd file, so this setting can be pointed directly to one of those backend files.
What is the 3rd column of the file used for?
(the 1st being username and
the 2nd being md5 hashed password)
Regards
Alex