"unexpected error: runtime error: integer divide by zero" doing subquery 

```
quantile_over_time(0.99, sum(rate(cortex_gw_request_duration_seconds_count{job=~"(.+)/cortex-gw", route="cortex-read"}[5m]))[24h:])
```

NB this doesn't happen with prometheus 2.8rc0.  Either we need to update vendor or we're missing some config.