Feature request: native config auto-reload support, similar to Prometheus `auto-reload-config`

Proposal

Alertmanager supports runtime reload via SIGHUP or POST /-/reload, but it does not appear to support automatic reload when the config file is updated on disk.

This is particularly inconvenient in Kubernetes environments where Alertmanager configuration is managed via ConfigMap or Secret. Even when a mounted file is updated automatically, Alertmanager does not pick up the change on its own, so users still need an external reloader sidecar just to detect the file change and call /-/reload.

It would be very helpful if Alertmanager had a native auto-reload mechanism similar to Prometheus --enable-feature=auto-reload-config.

Reference: https://prometheus.io/docs/prometheus/latest/feature_flags/#auto-reload-config

Current behavior

1. Start Alertmanager with a config file:

alertmanager --config.file=/etc/alertmanager/alertmanager.yml --web.listen-address=:9093

2. Update /etc/alertmanager/alertmanager.yml on disk.
3. Alertmanager does not reload the updated config automatically.
4. Manually trigger a reload:

curl -X POST http://localhost:9093/-/reload

5. Only after step 4 is the updated config applied.

Expected behavior

Alertmanager optionally supports automatic reload when --config.file changes on disk, similar to Prometheus --enable-feature=auto-reload-config.

This would be especially useful for Kubernetes deployments where configuration is managed via ConfigMap or Secret and updated through mounted files.

Environment

• Alertmanager version: v0.32.0