* Expose config metrics

Config metrics are not currently not exposed
by the exporter. These metrics are used in some
existing dashboards.

Config metrics being exposed:
- max_client_conn: pgbouncer_config_max_client_conn
- max_user_connections: pgbouncer_config_max_user_connections

Signed-off-by: kenneth-duffel <93927344+kenneth-duffel@users.noreply.github.com>
Signed-off-by: kenneth-duffel <kenneth@duffel.com>