address feedback

spec/datadog/integration_spec.rb

@@ -22,12 +22,12 @@ def wait_for_tracer_sent
     end
 
     context 'for threads' do
-      let(:original_threads) { Thread.list }
+      let!(:original_threads) { Thread.list }
       let(:start_tracer) do
         Datadog::Tracing.trace('test.op') {}
 
         new_threads = Thread.list - original_threads
-        tracer_threads.concat(new_threads.map(&:object_id))
+        tracer_threads.concat(new_threads)
       end
       let(:tracer_threads) { [] }
 
@@ -43,10 +43,10 @@ def inspect_threads(threads)
 
         shutdown
 
-        post_shutdown_threads = Thread.list.map(&:object_id)
+        post_shutdown_threads = Thread.list
 
         expect(post_shutdown_threads & tracer_threads).to be_empty,
-          "Tracer threads not terminated: #{inspect_threads(Thread.list)}"
+          "Tracer threads not terminated: #{inspect_threads(post_shutdown_threads)}"
       end
     end