Unify transactions in active_record subscriber

Issue https://gitlab.com/gitlab-org/gitlab/-/issues/323163

lib/gitlab/metrics/background_transaction.rb

@@ -3,7 +3,7 @@
 module Gitlab
   module Metrics
     class BackgroundTransaction < Transaction
-      # Separate web transaction instance and web transaction instance
+      # Separate web transaction instance and background transaction instance
       BACKGROUND_THREAD_KEY = :_gitlab_metrics_background_transaction
       BACKGROUND_BASE_LABEL_KEYS = %i(endpoint_id feature_category).freeze
 
@@ -33,9 +33,7 @@ module Gitlab
       end
 
       def labels
-        return @labels if @labels
-
-        @labels = {
+        @labels ||= {
           endpoint_id: current_context&.get_attribute(:caller_id),
           feature_category: current_context&.get_attribute(:feature_category)
         }

lib/gitlab/metrics/subscribers/active_record.rb

@@ -61,27 +61,19 @@ module Gitlab
         end
 
         def increment(counter)
-          web_transaction&.increment("gitlab_transaction_#{counter}_total".to_sym, 1)
-          background_transaction&.increment("gitlab_transaction_#{counter}_total".to_sym, 1)
+          current_transaction&.increment("gitlab_transaction_#{counter}_total".to_sym, 1)
 
           Gitlab::SafeRequestStore[counter] = Gitlab::SafeRequestStore[counter].to_i + 1
         end
 
         def observe(histogram, event)
-          web_transaction&.observe(histogram, event.duration / 1000.0) do
+          current_transaction&.observe(histogram, event.duration / 1000.0) do
             buckets DURATION_BUCKET
           end
-          background_transaction&.observe(histogram, event.duration / 1000.0) do
-            buckets DURATION_BUCKET
-          end
-        end
-
-        def web_transaction
-          ::Gitlab::Metrics::WebTransaction.current
         end
 
-        def background_transaction
-          ::Gitlab::Metrics::BackgroundTransaction.current
+        def current_transaction
+          ::Gitlab::Metrics::WebTransaction.current || ::Gitlab::Metrics::BackgroundTransaction.current
         end
       end
     end

spec/lib/gitlab/database_spec.rb

@@ -459,7 +459,7 @@ RSpec.describe Gitlab::Database do
       events
     end
 
-    context 'without a tranastion block' do
+    context 'without a transaction block' do
       it 'does not publish a transaction event' do
         events = subscribe_events do
           User.first

spec/support/shared_examples/metrics/active_record_subscriber_shared_examples.rb

@@ -83,16 +83,23 @@ end
 
 RSpec.shared_examples 'record ActiveRecord metrics' do |db_role|
   context 'when both web and background transaction are available' do
+    let(:transaction) { double('Gitlab::Metrics::WebTransaction') }
+    let(:background_transaction) { double('Gitlab::Metrics::WebTransaction') }
+
     before do
       allow(::Gitlab::Metrics::WebTransaction).to receive(:current)
-        .and_return(nil)
+        .and_return(transaction)
       allow(::Gitlab::Metrics::BackgroundTransaction).to receive(:current)
-        .and_return(nil)
+        .and_return(background_transaction)
+      allow(transaction).to receive(:increment)
+      allow(transaction).to receive(:observe)
     end
 
-    it 'does not capture the metrics' do
-      expect_any_instance_of(Gitlab::Metrics::Transaction).not_to receive(:increment) # rubocop:disable RSpec/AnyInstanceOf
-      expect_any_instance_of(Gitlab::Metrics::Transaction).not_to receive(:observe) # rubocop:disable RSpec/AnyInstanceOf
+    it_behaves_like 'record ActiveRecord metrics in a metrics transaction', db_role
+
+    it 'captures the metrics for web only' do
+      expect(background_transaction).not_to receive(:observe)
+      expect(background_transaction).not_to receive(:increment)
 
       subscriber.sql(event)
     end