Improve loading time for large number of events

We have noted large compute time spent on `#define_method` by
`BatchLoader` and this has contribute to the slowness on large CSV
import queries. This change will hopefully reduce these up-front costs
and as a result, reduce total processing time.

More information about `replace_methods` option can be found at
https://github.com/exAspArk/batch-loader#replacing-methods

app/models/audit_event.rb

@@ -65,8 +65,8 @@ class AuditEvent < ApplicationRecord
   end
 
   def lazy_author
-    BatchLoader.for(author_id).batch(default_value: default_author_value) do |author_ids, loader|
-      User.where(id: author_ids).find_each do |user|
+    BatchLoader.for(author_id).batch(default_value: default_author_value, replace_methods: false) do |author_ids, loader|
+      User.select(:id, :name, :username).where(id: author_ids).find_each do |user|
         loader.call(user.id, user)
       end
     end

ee/app/models/ee/audit_event.rb

@@ -47,7 +47,7 @@ module EE
     def lazy_entity
       BatchLoader.for(entity_id)
         .batch(
-          key: entity_type, default_value: ::Gitlab::Audit::NullEntity.new
+          key: entity_type, default_value: ::Gitlab::Audit::NullEntity.new, replace_methods: false
         ) do |ids, loader, args|
           model = Object.const_get(args[:key], false)
           model.where(id: ids).find_each { |record| loader.call(record.id, record) }

ee/changelogs/unreleased/246742-reduce-batch-loader-footprint-audit-events.yml

+---
+title: Reduce load time of large number of audit events
+merge_request: 43248
+author:
+type: performance