that was easy

presets/digitalocean_discovery.Caddyfile

+:5432 {
+	ssl
+
+	discovery digitalocean {$PGGAT_DO_API_KEY}
+}

presets/google_cloud_sql_discovery.Caddyfile

+:5432 {
+	ssl
+
+	discovery google_cloud_sql {$PGGAT_GC_PROJECT} {$PGGAT_GC_AUTH_USER:pggat} {$PGGAT_GC_AUTH_PASSWORD}
+}

presets/zalando_operator_discovery.Caddyfile

+:5432 {
+	ssl
+
+	discovery zalando_operator {$POSTGRES_OPERATOR_CONFIGURATION_OBJECT}
+}