Record and report time for trigger events

Sometimes, e.g. during reconfiguration, it can take quite some time
between the trigger event and when a change is enqueued.

This change allows tracking the time it takes from receiving the event
until it is processed by the scheduler.

Change-Id: I347acf56bc8d7671d96f6be444c71902563684be

zuul/driver/mqtt/mqttreporter.py

@@ -43,6 +43,7 @@ class MQTTReporter(BaseReporter):
             'ref': getattr(item.change, 'ref', ''),
             'message': self._formatItemReport(
                 item, with_jobs=False),
+            'trigger_time': item.event.timestamp,
             'enqueue_time': item.enqueue_time,
             'buildset': {
                 'uuid': item.current_build_set.uuid,