MetricConfigurations cache on MezuroConfiguration's show #46.

Closes #46.

Signed-off-by: Beatriz Rezener beatrizrezener@gmail.com
Signed-off-by: João Gabriel joaogabrieldebrittoesilva@gmail.com

This looks stupid, but will make a lot of difference for the person reading this code in the future: can you instead of calling the key for the cache just ```#{@mezuro_configuration.id}_metrics``` call it ```#{@mezuro_configuration.id}_metric_configurations```?

Since Metric is a type as well it can get really confusing :)