alertmanager: send_resolved does nothing on Pagerduty receiver

Per the Pagerduty receiver docs, we have our configured with send_resolved: true.

Experimentally, it doesn’t seem to do anything. I manually stopped a service in a way that would trigger a pagerduty alert from the alertmanager, and after the alert fired and I got the page, I restored the service and watched the alert clear from Prometheus and the Alertmanager. The Pagerduty incident neither changed nor recorded having received any additional input.

I would have expected send_resolved to put the incident in the “resolved” state.

(h/t to @stapelberg’s Pushover feature request for reminding me to verify this behavior)