Add wiki_size and storage_size to NamespaceStatistics

Add columns `wiki_size` and `storage_size` to `NamespaceStatistics`
in order to track namespace's storage use.

changelogs/unreleased/230465-fj-add-columns-to-track-group-wiki-storage.yml

+---
+title: Add wiki_size and storage_size to NamespaceStatistics
+merge_request: 54786
+author:
+type: added

db/migrate/20210222070356_add_storage_size_to_namespace_statistics.rb

+# frozen_string_literal: true
+
+class AddStorageSizeToNamespaceStatistics < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    with_lock_retries do
+      add_column :namespace_statistics, :storage_size, :bigint, default: 0, null: false
+    end
+  end
+
+  def down
+    with_lock_retries do
+      remove_column :namespace_statistics, :storage_size
+    end
+  end
+end

db/migrate/20210222070413_add_wiki_size_to_namespace_statistics.rb

+# frozen_string_literal: true
+
+class AddWikiSizeToNamespaceStatistics < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    with_lock_retries do
+      add_column :namespace_statistics, :wiki_size, :bigint, default: 0, null: false
+    end
+  end
+
+  def down
+    with_lock_retries do
+      remove_column :namespace_statistics, :wiki_size
+    end
+  end
+end

db/schema_migrations/20210222070356

+2aa618cdbb6e55c3af1fa144ae3f3bd0d9d5400706db337301d2d165ba789ef0
\ No newline at end of file

db/schema_migrations/20210222070413

+e13856c6f65d86dae9a3268f91189bb90af236c555d2f645c6ba4c600c996cba
\ No newline at end of file

db/structure.sql

@@ -14373,7 +14373,9 @@ CREATE TABLE namespace_statistics (
     id integer NOT NULL,
     namespace_id integer NOT NULL,
     shared_runners_seconds integer DEFAULT 0 NOT NULL,
-    shared_runners_seconds_last_reset timestamp without time zone
+    shared_runners_seconds_last_reset timestamp without time zone,
+    storage_size bigint DEFAULT 0 NOT NULL,
+    wiki_size bigint DEFAULT 0 NOT NULL
 );
 
 CREATE SEQUENCE namespace_statistics_id_seq