Authentication for admin database

[erthalion]

Hi,

From what I understand, if stats_users specified in configuration, it needs
to be present in auth_file to be able to connect to the admin db. This is the
case even if stats_users could be authenticated via auth_user +
auth_query for all other "regular" databases. Looks like it happens because
the admin db is a special "fake" database without an auth_user. If pgbouncer
would be able to authenticate stats_users connecting to the admin db using
auth_query as well, this will be helpful because one doesn't need to maintain
another set of credentials for a monitoring user.

From the first glance there are just few bits missing to implement this. E.g. a
default_auth_db to connect to for authentication purposes in case of admin
db. Does this implementation make sense?

This idea also partially intersects with #569, but instead of restricting this
patch for admin databases it handles them differently.

[erthalion]

Looks like appveyor tests are failing, because postgres version has changed since the last build (from postgresql-12.4-1 to postgresql-13.1-1). Probably something needs to be adjusted in the postgres config, looking into this, but I guess it's not relevant for this particular patch:

2021-02-21 12:37:45.633 UTC [5280] LOG:  listening on IPv6 address "::1", port 6666
2021-02-21 12:37:45.633 UTC [5280] LOG:  listening on IPv4 address "127.0.0.1", port 6666
2021-02-21 12:37:45.648 UTC [5280] FATAL:  could not create lock file "/tmp/.s.PGSQL.6666.lock": No such file or directory
2021-02-21 12:37:45.648 UTC [5280] LOG:  database system is shut down
 stopped waiting
pg_ctl: could not start server
Examine the log output.