Use concurrent helpers in migration to change type

Use the concurrent helpers recommended by our migration guide to change
type of historical_data.date to timestamptz.

db/migrate/20201022094845_change_historical_data_date_type.rb

 # frozen_string_literal: true
 
 class ChangeHistoricalDataDateType < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
   DOWNTIME = false
 
-  def up
-    # historical_data only contains 1059 rows in gitlab.com. A new row is added
-    # once a day (at 12) by a cronjob (HistoricalDataWorker).
-    # Self-managed GitLab instances will probably have even less data, since gitlab.com
-    # is one of the oldest GitLab instances.
+  disable_ddl_transaction!
 
-    # We're using `Time.zone.tzinfo.name` here because the sidekiq-cron gem
-    # evaluates crons against the timezone configured in Rails.
+  def up
     execute(
       <<SQL
-      ALTER TABLE historical_data ALTER COLUMN date TYPE timestamptz USING ((date + '12:00'::time) AT TIME ZONE '#{Time.zone.tzinfo.name}');
+      create or replace function change_historical_date_to_timetamptz(date)
+        returns timestamptz
+        language sql
+        as
+      $$
+        SELECT ($1 + '12:00'::time) AT TIME ZONE '#{Time.zone&.tzinfo&.name || "Etc/UTC"}'
+      $$
 SQL
     )
+
+    change_column_type_concurrently(:historical_data, :date, :timestamptz, type_cast_function: "change_historical_date_to_timetamptz")
+
+    execute("DROP FUNCTION IF EXISTS change_historical_date_to_timetamptz")
   end
 
   def down
-    change_column :historical_data, :date, :date
+    undo_change_column_type_concurrently(:historical_data, :date)
   end
 end

db/post_migrate/20201024103311_change_historical_data_date_type_cleanup.rb

+# frozen_string_literal: true
+
+class ChangeHistoricalDataDateTypeCleanup < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    cleanup_concurrent_column_type_change(:historical_data, :date)
+  end
+
+  def down
+    undo_cleanup_concurrent_column_type_change(:historical_data, :date, :date)
+  end
+end

db/schema_migrations/20201024103311

+7a1719eb6975d731f89ce7e269868956d220e8f63dfd1739371ae2ddfd0e2473
\ No newline at end of file

db/structure.sql

@@ -12699,10 +12699,11 @@ CREATE TABLE group_wiki_repositories (
 
 CREATE TABLE historical_data (
     id integer NOT NULL,
-    date timestamp with time zone NOT NULL,
     active_user_count integer,
     created_at timestamp without time zone,
-    updated_at timestamp without time zone
+    updated_at timestamp without time zone,
+    date timestamp with time zone,
+    CONSTRAINT check_8f3547750e CHECK ((date IS NOT NULL))
 );
 
 CREATE SEQUENCE historical_data_id_seq