Refactor background transaction tests

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

spec/lib/gitlab/metrics/background_transaction_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Metrics::BackgroundTransaction do
+  let(:transaction) { described_class.new }
+  let(:prometheus_metric) { instance_double(Prometheus::Client::Metric, base_labels: {}) }
+
+  before do
+    allow(described_class).to receive(:prometheus_metric).and_return(prometheus_metric)
+  end
+
+  describe '#run' do
+    it 'yields the supplied block' do
+      expect { |b| transaction.run(&b) }.to yield_control
+    end
+
+    it 'stores the transaction in the current thread' do
+      transaction.run do
+        expect(Thread.current[described_class::BACKGROUND_THREAD_KEY]).to eq(transaction)
+      end
+    end
+
+    it 'removes the transaction from the current thread upon completion' do
+      transaction.run { }
+
+      expect(Thread.current[described_class::BACKGROUND_THREAD_KEY]).to be_nil
+    end
+  end
+
+  describe '#labels' do
+    it 'provides labels with endpoint_id and feature_category' do
+      Labkit::Context.with_context(feature_category: 'projects', caller_id: 'TestWorker') do
+        expect(transaction.labels).to eq({ endpoint_id: 'TestWorker', feature_category: 'projects' })
+      end
+    end
+  end
+
+  RSpec.shared_examples 'metric with labels' do |metric_method|
+    it 'measures with correct labels and value' do
+      value = 1
+      expect(prometheus_metric).to receive(metric_method).with({ endpoint_id: 'TestWorker', feature_category: 'projects' }, value)
+
+      Labkit::Context.with_context(feature_category: 'projects', caller_id: 'TestWorker') do
+        transaction.send(metric_method, :test_metric, value)
+      end
+    end
+  end
+
+  describe '#increment' do
+    let(:prometheus_metric) { instance_double(Prometheus::Client::Counter, :increment, base_labels: {}) }
+
+    it_behaves_like 'metric with labels', :increment
+  end
+
+  describe '#set' do
+    let(:prometheus_metric) { instance_double(Prometheus::Client::Gauge, :set, base_labels: {}) }
+
+    it_behaves_like 'metric with labels', :set
+  end
+
+  describe '#observe' do
+    let(:prometheus_metric) { instance_double(Prometheus::Client::Histogram, :observe, base_labels: {}) }
+
+    it_behaves_like 'metric with labels', :observe
+  end
+end

spec/lib/gitlab/sidekiq_middleware/server_metrics_spec.rb

@@ -113,6 +113,14 @@ RSpec.describe Gitlab::SidekiqMiddleware::ServerMetrics do
             expect { |b| subject.call(worker, job, :test, &b) }.to yield_control.once
           end
 
+          it 'calls BackgroundTransaction' do
+            expect_next_instance_of(Gitlab::Metrics::BackgroundTransaction) do |instance|
+              expect(instance).to receive(:run)
+            end
+
+            subject.call(worker, job, :test) {}
+          end
+
           it 'sets queue specific metrics' do
             expect(running_jobs_metric).to receive(:increment).with(labels, -1)
             expect(running_jobs_metric).to receive(:increment).with(labels, 1)

spec/support/shared_examples/metrics/active_record_subscriber_shared_examples.rb

@@ -99,26 +99,30 @@ RSpec.shared_examples 'record ActiveRecord metrics' do |db_role|
   end
 
   context 'when web transaction is available' do
-    let(:transaction) { double('Gitlab::Metrics::WebTransaction', increment: nil, observe: nil) }
+    let(:transaction) { double('Gitlab::Metrics::WebTransaction') }
 
     before do
       allow(::Gitlab::Metrics::WebTransaction).to receive(:current)
         .and_return(transaction)
       allow(::Gitlab::Metrics::BackgroundTransaction).to receive(:current)
         .and_return(nil)
+      allow(transaction).to receive(:increment)
+      allow(transaction).to receive(:observe)
     end
 
     it_behaves_like 'record ActiveRecord metrics in a metrics transaction', db_role
   end
 
   context 'when background transaction is available' do
-    let(:transaction) { double('Gitlab::Metrics::BackgroundTransaction', increment: nil, observe: nil) }
+    let(:transaction) { double('Gitlab::Metrics::BackgroundTransaction') }
 
     before do
       allow(::Gitlab::Metrics::WebTransaction).to receive(:current)
         .and_return(nil)
       allow(::Gitlab::Metrics::BackgroundTransaction).to receive(:current)
         .and_return(transaction)
+      allow(transaction).to receive(:increment)
+      allow(transaction).to receive(:observe)
     end
 
     it_behaves_like 'record ActiveRecord metrics in a metrics transaction', db_role