Include subtransactions created by ActiveRecord in counters

This commit includes subtransactions created directly by ActiveRecord in
our Prometheus counter to detect subtransactions generated by migrations
too.

app/models/application_record.rb

@@ -104,19 +104,6 @@ class ApplicationRecord < ActiveRecord::Base
     enum(enum_mod.key => values)
   end
 
-  def self.transaction(**options, &block)
-    if options[:requires_new] && track_subtransactions?
-      ::Gitlab::Database::Metrics.subtransactions_increment(self.name)
-    end
-
-    super(**options, &block)
-  end
-
-  def self.track_subtransactions?
-    ::Feature.enabled?(:active_record_subtransactions_counter, type: :ops, default_enabled: :yaml) &&
-      connection.transaction_open?
-  end
-
   def self.cached_column_list
     self.column_names.map { |column_name| self.arel_table[column_name] }
   end

lib/gitlab/database.rb

@@ -210,10 +210,19 @@ module Gitlab
         # A monkeypatch over ActiveRecord::Base.transaction.
         # It provides observability into transactional methods.
         def transaction(**options, &block)
+          if options[:requires_new] && gitlab_track_subtransactions?
+            ::Gitlab::Database::Metrics.subtransactions_increment(self.name)
+          end
+
           ActiveSupport::Notifications.instrument('transaction.active_record', { connection: connection }) do
             super(**options, &block)
           end
         end
+
+        def gitlab_track_subtransactions?
+          ::Feature.enabled?(:active_record_subtransactions_counter, type: :ops, default_enabled: :yaml) &&
+            connection.transaction_open?
+        end
       end
     end
 

spec/models/application_record_spec.rb

@@ -164,6 +164,23 @@ RSpec.describe ApplicationRecord do
         end
       end
     end
+
+    # rubocop:disable Database/MultipleDatabases
+    it 'increments a counter when a transaction is created in ActiveRecord' do
+      expect(described_class.connection.transaction_open?).to be false
+
+      expect(::Gitlab::Database::Metrics)
+        .to receive(:subtransactions_increment)
+        .with('ActiveRecord::Base')
+        .once
+
+      ActiveRecord::Base.transaction do
+        ActiveRecord::Base.transaction(requires_new: true) do
+          expect(ActiveRecord::Base.connection.transaction_open?).to be true
+        end
+      end
+    end
+    # rubocop:enable Database/MultipleDatabases
   end
 
   describe '.with_fast_read_statement_timeout' do