Schedule background migration to populate pages metadata

Call MigratePagesMetadata for batches of projects to populate
`project_pages_metadata` table.

Related to https://gitlab.com/gitlab-org/gitlab/issues/28781.

changelogs/unreleased/28781-migrate-pages-metadata-in-background.yml

+---
+title: Schedule background migration to populate pages metadata
+merge_request: 17993
+author:
+type: added

db/post_migrate/20191002031332_schedule_pages_metadata_migration.rb

+# frozen_string_literal: true
+
+class SchedulePagesMetadataMigration < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  BATCH_SIZE = 10_000
+  MIGRATION = 'MigratePagesMetadata'
+
+  disable_ddl_transaction!
+
+  class Project < ActiveRecord::Base
+    include ::EachBatch
+
+    self.table_name = 'projects'
+  end
+
+  def up
+    say "Scheduling `#{MIGRATION}` jobs"
+
+    # At the time of writing there are ~10_669_292 records to be inserted for GitLab.com,
+    # batches of 10_000 with delay interval of 2 minutes gives us an estimate of close to 36 hours.
+    queue_background_migration_jobs_by_range_at_intervals(Project, MIGRATION, 2.minutes, batch_size: BATCH_SIZE)
+  end
+
+  def down
+    # no-op
+  end
+end

spec/migrations/schedule_pages_metadata_migration_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20191002031332_schedule_pages_metadata_migration')
+
+describe SchedulePagesMetadataMigration, :migration, :sidekiq do
+  let(:namespaces) { table(:namespaces) }
+  let(:projects) { table(:projects) }
+
+  before do
+    stub_const("#{described_class.name}::BATCH_SIZE", 1)
+
+    namespaces.create!(id: 11, name: 'gitlab', path: 'gitlab-org')
+    projects.create!(id: 111, namespace_id: 11, name: 'Project 111')
+    projects.create!(id: 114, namespace_id: 11, name: 'Project 114')
+  end
+
+  it 'schedules pages metadata migration' do
+    Sidekiq::Testing.fake! do
+      Timecop.freeze do
+        migrate!
+
+        expect(described_class::MIGRATION).to be_scheduled_delayed_migration(2.minutes, 111, 111)
+        expect(described_class::MIGRATION).to be_scheduled_delayed_migration(4.minutes, 114, 114)
+        expect(BackgroundMigrationWorker.jobs.size).to eq(2)
+      end
+    end
+  end
+end