Add management event queue length to status json

We grew another event queue, but did not expose it.

Change-Id: Iad68f6b193f691d43de2c7fef0645c3c1f6f161f
diff --git a/zuul/scheduler.py b/zuul/scheduler.py
index e5924f8..07ca258 100644
--- a/zuul/scheduler.py
+++ b/zuul/scheduler.py
@@ -939,6 +939,9 @@
         data['result_event_queue'] = {}
         data['result_event_queue']['length'] = \
             self.result_event_queue.qsize()
+        data['management_event_queue'] = {}
+        data['management_event_queue']['length'] = \
+            self.management_event_queue.qsize()
 
         if self.last_reconfigured:
             data['last_reconfigured'] = self.last_reconfigured * 1000