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

You could interpolate ```params[:mezuro_configuration_id]``` directly here instead of creating this new var and setting it on ```metric_configuration_id```