Add caching layer to metrics (#17617)

app/lib/admin/metrics/dimension/space_usage_dimension.rb

@@ -8,12 +8,12 @@ class Admin::Metrics::Dimension::SpaceUsageDimension < Admin::Metrics::Dimension
     'space_usage'
   end
 
-  def data
+  protected
+
+  def perform_query
     [postgresql_size, redis_size, media_size]
   end
 
-  private
-
   def postgresql_size
     value = ActiveRecord::Base.connection.execute('SELECT pg_database_size(current_database())').first['pg_database_size']