Add metric to report SRE Overview summary of alerts

[stephen-soltesz]

We want a summary of alerts that fired over the last week to review at the ops tactical meetings.

We thought that githubreceiver_alerts_total would do the trick, but it has a bug and does not accurately report firing alerts. It should be fixed or removed.

[findmyname666]

Information about the alerts and their state is already present in the Prometheus.