Add a test case to improve ci queue metrics coverage

spec/services/ci/register_job_service_spec.rb

@@ -837,6 +837,7 @@ module Ci
 
           it 'counts job queuing time histogram with expected labels' do
             allow(attempt_counter).to receive(:increment)
+
             expect(job_queue_duration_seconds).to receive(:observe)
               .with({ shared_runner: expected_shared_runner,
                       jobs_running_for_project: expected_jobs_running_for_project_third_job,
@@ -888,6 +889,16 @@ module Ci
           it_behaves_like 'metrics collector'
         end
 
+        context 'when max running jobs bucket size is exceeded' do
+          before do
+            stub_const('Gitlab::Ci::Queue::Metrics::JOBS_RUNNING_FOR_PROJECT_MAX_BUCKET', 1)
+          end
+
+          let(:expected_jobs_running_for_project_third_job) { '1+' }
+
+          it_behaves_like 'metrics collector'
+        end
+
         context 'when pending job with queued_at=nil is used' do
           before do
             pending_job.update!(queued_at: nil)