check_pgb_pool generates duplicate metrics per database

[mayrstefan]

The check_pgb_pool function generates metrics per database. But PgBouncer SHOW POOLS generates a tuple of database and user per row.

So if we have a database exampledb and SHOW POOLS has data for userA and userB we get the metric for exampledb twice. So even if those are two datapoints you might get only one metric in your graphing tool.