Never report negative idle time

We calculate idle time by subtracting two separately-obtained values,
and apparently it's possible for the CPU-time clock to tick a
millisecond ahead of the monotonic clock.

I think it's safe to ignore that potential difference in general, but
even at the extreme of CPU activity, I'm reasonably confident time
doesn't move backwards.

activesupport/lib/active_support/notifications/instrumenter.rb

@@ -155,7 +155,8 @@ def cpu_time
       # Returns the idle time time (in milliseconds) passed since the call to
       # +start!+ and the call to +finish!+
       def idle_time
-        duration - cpu_time
+        diff = duration - cpu_time
+        diff > 0.0 ? diff : 0.0
       end
 
       # Returns the number of allocations made since the call to +start!+ and

activesupport/test/log_subscriber_test.rb

@@ -111,7 +111,7 @@ def test_event_attributes
       assert_operator event.allocations, :>, 0
     end
     assert_operator event.duration, :>, 0
-    assert_operator event.idle_time, :>, 0
+    assert_operator event.idle_time, :>=, 0
   end
 
   def test_does_not_send_the_event_if_it_doesnt_match_the_class

activesupport/test/notifications_test.rb

@@ -37,7 +37,7 @@ def test_subscribe_events
       assert event, "should have an event"
       assert_operator event.allocations, :>, 0
       assert_operator event.cpu_time, :>, 0
-      assert_operator event.idle_time, :>, 0
+      assert_operator event.idle_time, :>=, 0
       assert_operator event.duration, :>, 0
     end