add sleep

Author: johha

spec/unit/lib/delayed_job_plugins/delayed_jobs_metrics_spec.rb

@@ -21,6 +21,7 @@
         worker = Delayed::Worker.new
         worker.name = 'test_worker'
         worker.work_off(1)
+        sleep(2)
 
         expect(prometheus).to have_received(:update_histogram_metric).with(
           :cc_job_pickup_delay_seconds,