Make dashboard a top level key

- Since each yml file can hold only one dashboard.

db/importers/common_metrics_importer.rb

@@ -38,7 +38,7 @@ module Importers
     attr_reader :content
 
     def initialize(filename = 'common_metrics.yml')
-      @content = YAML.load_file(Rails.root.join('config', 'prometheus', filename)).first
+      @content = YAML.load_file(Rails.root.join('config', 'prometheus', filename))
     end
 
     def execute