monitoring: base alert_count only on state=firing alerts
Created by: bobheadxi
ALERTS can be either "pending" or "firing", and we only care about the latter for alert_count
I believe this is causing us to over-count alerts - see: https://sourcegraph.slack.com/archives/CMBA8F926/p1595742704207800, caused by https://github.com/sourcegraph/sourcegraph/pull/12395
