Show warning on decoder latency spikes

Stackdriver dashboard showing latency spikes: https://app.google.stackdriver.com/dashboards/8747181677664918117?project=moz-fx-data-ingesti-prod-579d.

whd's comment:
```
It would be relatively easy to pull this information from stackdriver, with the caveat that the beam jobs do change name fairly often so you would have to list+filter when pulling the latency information.
```