autotest: add a suite_scheduler tick counter

The tick counter is incremented at the end of each tick loop (to ensure
that if the suite_scheduler is crashlooping, we'll see that as a 0
rate).

BUG=None
TEST=None

Change-Id: Ib2bee964681cae68591d06d4f09468e5a544f83d
Reviewed-on: https://chromium-review.googlesource.com/480997
Commit-Ready: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>
diff --git a/site_utils/suite_scheduler/driver.py b/site_utils/suite_scheduler/driver.py
index 7651d81..e41e8e2 100644
--- a/site_utils/suite_scheduler/driver.py
+++ b/site_utils/suite_scheduler/driver.py
@@ -139,6 +139,8 @@
             task.TotMilestoneManager().refresh()
             time.sleep(self._LOOP_INTERVAL_SECONDS)
             self.RereadAndReprocessConfig(config, mv)
+            metrics.Counter('chromeos/autotest/suite_scheduler/'
+                            'handle_events_tick').increment()
 
 
     @staticmethod