Hi Akito,
Generally the answer to " Is there a way to adjust Fly.io’s Prometheus configuration to provide valid data for a self-hosted AlertManager instance?" is “No”. As you’re aware, alerting rules are set on Prometheus, not on Alertmanager, and there’s no way to set custom rules on Fly Prometheus.
You can set up a self-hosted Prometheus whose main goal is to allow you to set the alerting rules and fire alerts on Alertmanager, I describe the setup here: Self-managed monitoring and alerting on Fly.io with Prometheus + Alertmanager.
Let me know if that helps!
- Daniel