Speed up Elastic AddNewDataToMergeRequestsDocuments migration

This migration was just introduced in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/59836 .

After watching this migration running in production we can see that we
can make it run faster. The numbers here also match what we used in
`AddPermissionsDataToNotesDocuments` so should be safe.

I've also changed it to use the `ProcessInitialBookkeepingService` which
is basically just moving the work into a different queue. We have 2
queues for indexing work in Elasticsearch and the "initial" queue is for
newly created projects/imported projects. It gets much less traffic than
the so called "incremental" queue and therefore it's safer to put
backfill migrations into this queue. We actually should have used this
queue to begin with but switching it now is perfectly fine.

Changelog: changed

ee/changelogs/unreleased/322788-speed-up-backfill-mr-permissions-elasticsearch-migration.yml

+---
+title: Speed up Elastic AddNewDataToMergeRequestsDocuments migration
+merge_request: 60563
+author:
+type: changed

ee/elastic/migrate/20210421140400_add_new_data_to_merge_requests_documents.rb

@@ -2,9 +2,9 @@
 
 class AddNewDataToMergeRequestsDocuments < Elastic::Migration
   batched!
-  throttle_delay 5.minutes
+  throttle_delay 3.minutes
 
-  QUERY_BATCH_SIZE = 5000
+  QUERY_BATCH_SIZE = 6000
   UPDATE_BATCH_SIZE = 100
 
   def migrate
@@ -38,7 +38,7 @@ class AddNewDataToMergeRequestsDocuments < Elastic::Migration
     end
 
     document_references.each_slice(UPDATE_BATCH_SIZE) do |refs|
-      Elastic::ProcessBookkeepingService.track!(*refs)
+      Elastic::ProcessInitialBookkeepingService.track!(*refs)
     end
 
     log "Adding visibility_level field to merge_requests documents is completed for batch of #{document_references.size} documents"

ee/spec/elastic/migrate/20210421140400_add_new_data_to_merge_requests_documents_spec.rb

@@ -20,7 +20,7 @@ RSpec.describe AddNewDataToMergeRequestsDocuments, :elastic, :sidekiq_inline do
   describe 'migration_options' do
     it 'has migration options set', :aggregate_failures do
       expect(migration.batched?).to be_truthy
-      expect(migration.throttle_delay).to eq(5.minutes)
+      expect(migration.throttle_delay).to eq(3.minutes)
     end
   end
 
@@ -33,7 +33,7 @@ RSpec.describe AddNewDataToMergeRequestsDocuments, :elastic, :sidekiq_inline do
       end
 
       it 'does not modify data', :aggregate_failures do
-        expect(::Elastic::ProcessBookkeepingService).not_to receive(:track!)
+        expect(::Elastic::ProcessInitialBookkeepingService).not_to receive(:track!)
 
         subject
       end
@@ -46,7 +46,7 @@ RSpec.describe AddNewDataToMergeRequestsDocuments, :elastic, :sidekiq_inline do
 
       it 'updates all merge_request documents' do
         # track calls are batched in groups of 100
-        expect(::Elastic::ProcessBookkeepingService).to receive(:track!).once do |*tracked_refs|
+        expect(::Elastic::ProcessInitialBookkeepingService).to receive(:track!).once do |*tracked_refs|
           expect(tracked_refs.count).to eq(3)
         end
 
@@ -58,7 +58,7 @@ RSpec.describe AddNewDataToMergeRequestsDocuments, :elastic, :sidekiq_inline do
         add_visibility_level_for_merge_requests(merge_requests[1..-1])
 
         expected = [Gitlab::Elastic::DocumentReference.new(MergeRequest, merge_request.id, merge_request.es_id, merge_request.es_parent)]
-        expect(::Elastic::ProcessBookkeepingService).to receive(:track!).with(*expected).once
+        expect(::Elastic::ProcessInitialBookkeepingService).to receive(:track!).with(*expected).once
 
         subject
       end
@@ -67,7 +67,7 @@ RSpec.describe AddNewDataToMergeRequestsDocuments, :elastic, :sidekiq_inline do
         stub_const("#{described_class}::QUERY_BATCH_SIZE", 2)
         stub_const("#{described_class}::UPDATE_BATCH_SIZE", 1)
 
-        expect(::Elastic::ProcessBookkeepingService).to receive(:track!).exactly(3).times.and_call_original
+        expect(::Elastic::ProcessInitialBookkeepingService).to receive(:track!).exactly(3).times.and_call_original
 
         # cannot use subject in spec because it is memoized
         migration.migrate