PERF: Update Group#user_count counter cache outside DB transaction (#19256)

While load testing our user creation code path in production, we
identified that executing the DB statement to update the `Group#user_count` column within a
transaction is creating a bottleneck for us. This is because the
creation of a user and addition of the user to the relevant groups are
done in a transaction. When we execute the DB statement to update
`Group#user_count` for the relevant group, a row level lock is held
until the transaction completes. This row level lock acts like a global
lock when the server is creating users that will be added to the same
group in quick succession.

Instead of updating the counter cache within a transaction which the
default ActiveRecord `counter_cache` option does, we simply update the
counter cache outside of the committing transaction.

Co-authored-by: Rafael dos Santos Silva <xfalcox@gmail.com>

Co-authored-by: Rafael dos Santos Silva <xfalcox@gmail.com>

app/models/group_user.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class GroupUser < ActiveRecord::Base
-  belongs_to :group, counter_cache: "user_count"
+  belongs_to :group
   belongs_to :user
 
   after_save :update_title
@@ -15,6 +15,9 @@ class GroupUser < ActiveRecord::Base
   after_save :set_category_notifications
   after_save :set_tag_notifications
 
+  after_commit :increase_group_user_count, on: [:create]
+  after_commit :decrease_group_user_count, on: [:destroy]
+
   def self.notification_levels
     NotificationLevels.all
   end
@@ -186,6 +189,14 @@ class GroupUser < ActiveRecord::Base
       end
     end
   end
+
+  def increase_group_user_count
+    Group.increment_counter(:user_count, self.group_id)
+  end
+
+  def decrease_group_user_count
+    Group.decrement_counter(:user_count, self.group_id)
+  end
 end
 
 # == Schema Information