Add packages_size to ProjectStatistics

This new field will allow to keep track of the storage used by the
packages features, it provides also aggregation at namespace level.

db/schema.rb

@@ -2463,6 +2463,7 @@ ActiveRecord::Schema.define(version: 20190426180107) do
     t.bigint "packages_size"
     t.bigint "shared_runners_seconds", default: 0, null: false
     t.datetime "shared_runners_seconds_last_reset"
+    t.bigint "packages_size"
     t.index ["namespace_id"], name: "index_project_statistics_on_namespace_id", using: :btree
     t.index ["project_id"], name: "index_project_statistics_on_project_id", unique: true, using: :btree
   end

spec/models/project_statistics_spec.rb

@@ -124,7 +124,7 @@ describe ProjectStatistics do
   end
 
   describe '.increment_statistic' do
-    shared_examples 'a statistic that increases storage_size' do
+    shared_examples 'a statistic that also increases storage_size' do
       it 'increases the statistic by that amount' do
         expect { described_class.increment_statistic(project.id, stat, 13) }
           .to change { statistics.reload.send(stat) || 0 }
@@ -153,7 +153,7 @@ describe ProjectStatistics do
     context 'when the amount is 0' do
       it 'does not execute a query' do
         project
-        expect { described_class.increment_statistic(project.id, :build_artifacts_size, 0) }
+        expect { described_class.increment_statistic(project.id, stat, 0) }
           .not_to exceed_query_limit(0)
       end
     end