Enable by default automatic optimization of batched background migration

See https://gitlab.com/gitlab-org/gitlab/-/issues/329511.

Changelog: added

changelogs/unreleased/329511-enable-by-default-optimize-batched-migrations.yml

+---
+title: Enable by default automatic optimization of batched background migrations
+merge_request: 61317
+author:
+type: added

config/feature_flags/ops/optimize_batched_migrations.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/328817
 milestone: '13.12'
 type: ops
 group: group::database
-default_enabled: false
+default_enabled: true

doc/user/admin_area/monitoring/background_migrations.md

@@ -46,3 +46,36 @@ To disable it:
 ```ruby
 Feature.disable(:execute_batched_migrations_on_schedule)
 ```
+
+## Automatic batch size optimization **(FREE SELF)**
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/60133) in GitLab 13.12.
+> - [Deployed behind a feature flag](../../../user/feature_flags.md), disabled by default.
+> - [Enabled by default](https://gitlab.com/gitlab-org/gitlab/-/issues/329511) in GitLab 13.12.
+> - Enabled on GitLab.com.
+> - Recommended for production use.
+> - For GitLab self-managed instances, GitLab administrators can opt to [disable it](#enable-or-disable-automatic-batch-size-optimization). **(FREE SELF)**
+
+There can be [risks when disabling released features](../../../user/feature_flags.md#risks-when-disabling-released-features).
+Refer to this feature's version history for more details.
+
+To maximize throughput of batched background migrations (in terms of the number of tuples updated per time unit), batch sizes are automatically adjusted based on how long the previous batches took to complete.
+
+## Enable or disable automatic batch size optimization **(FREE SELF)**
+
+Automatic batch size optimization for Batched Background Migrations is under development but ready for production use.
+It is deployed behind a feature flag that is **enabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../../administration/feature_flags.md)
+can opt to disable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:optimize_batched_migrations)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:optimize_batched_migrations)
+```

lib/gitlab/database/background_migration/batch_optimizer.rb

@@ -40,7 +40,7 @@ module Gitlab
         end
 
         def optimize!
-          return unless Feature.enabled?(:optimize_batched_migrations, type: :ops)
+          return unless Feature.enabled?(:optimize_batched_migrations, type: :ops, default_enabled: :yaml)
 
           if multiplier = batch_size_multiplier
             migration.batch_size = (migration.batch_size * multiplier).to_i.clamp(ALLOWED_BATCH_SIZE)