[Monitoring] Make appengine crons emit metrics (#4572)

Appengine crons are currently not instrumented, so we cannot get
testcase metrics for the google 3 deployment. This PR fixes that.

Part of #4271

src/appengine/libs/handler.py

@@ -30,6 +30,7 @@
 from clusterfuzz._internal.config import local_config
 from clusterfuzz._internal.datastore import data_types
 from clusterfuzz._internal.google_cloud_utils import pubsub
+from clusterfuzz._internal.metrics import monitor
 from clusterfuzz._internal.system import environment
 from libs import access
 from libs import auth
@@ -96,11 +97,12 @@ def wrapper(self):
       if not self.is_cron():
         raise helpers.AccessDeniedError('You are not a cron.')
 
-      result = func(self)
-      if result is None:
-        return 'OK'
+      with monitor.wrap_with_monitoring():
+        result = func(self)
+        if result is None:
+          return 'OK'
 
-      return result
+        return result
 
     return wrapper