Fix master loop failure metric (#6217)
Fix master loop failure metric (#6217)

Summary:
- fixed `marathon_loop_master.failure` metric which was previously sent in any case (instead of failure case)
- added `marathon_loop_master.duration` to the metrics
gsantovena/marathon,mesosphere/marathon,gsantovena/marathon,mesosphere/marathon,gsantovena/marathon,mesosphere/marathon,gsantovena/marathon,gsantovena/marathon,mesosphere/marathon,mesosphere/marathon