Add alert for buildbot scheduler

alertserver/alerts.cfg

Index: alertserver/alerts.cfg
diff --git a/alertserver/alerts.cfg b/alertserver/alerts.cfg
index 148f10fbd1d1590357766bbfd39a7112f0bab1d1..355243b744874b334d3081350a206993dddcfb3b 100644
--- a/alertserver/alerts.cfg
+++ b/alertserver/alerts.cfg
@@ -772,6 +772,21 @@ auto-dismiss = true
 nag = "1h"
 
 #
+# Buildbot Scheduler
+#
+
+[[rule]]
+name = "Buildbot Scheduler Failing"
+message = "The buildbot scheduler has failed to schedule builds for the last 10 minutes."
+database = "skmetrics"
+query = "select mean(value) from liveness where app='buildbot_scheduler' and host='skia-build-scheduler' and \"name\"='time-since-last-successful-scheduling' and time > now() - 5m group by app, host"
+category = "infra"
+condition = "x > 10 * 60 * 1000 * 1000 * 1000"
+actions = ["Email(infra-alerts@skia.org)"]
+auto-dismiss = true
+nag = "1h"
+
+#
 # CTFE
 #