Make it possible to choose which metrics are collected from Postgresql

Currently there is no way to specify which metrics are collected from postgresql. If you enable ``extended`` you will get all of them, but there is no way to remove some of the metrics that you might not be interested in.

This patch adds a new option named ``metrics`` that you can use to specify the list of metrics you want to collect. If this setting is not present it'll use the existing ``basic``/``extended`` toggle. I've not remove this option in order to make this patch backward compatible and hopefully see it merged into master soon.

I thought for a moment doing some kind of fancy module introspection in order to avoid hardcoding ``metrics_registry`` but I think it doesn't worth and it's outside the scope of this patch.