Add posibility to divide metric to the small pieces

For example:
We can have more cloud volume types of for allocated storate metric.

db/migrate/20171013125651_add_sub_metric_to_chargeback_rate_detail.rb

@@ -0,0 +1,5 @@
+class AddSubMetricToChargebackRateDetail < ActiveRecord::Migration[5.0]
+  def change
+    add_column :chargeback_rate_details, :sub_metric, :string
+  end
+end

db/schema.yml

@@ -218,6 +218,7 @@ chargeback_rate_details:
 - chargeback_rate_detail_measure_id
 - chargeback_rate_detail_currency_id
 - chargeable_field_id
+- sub_metric
 chargeback_rates:
 - id
 - guid