Problem statement:

Currently, Alertmanager is capable of responding to responses from the server at https://github.com/prometheus/alertmanager/blob/7cdecbf6ee97951d3ba1430fae6598a1fe9defe1/notify/util.go#L292.

// GetFailureReasonFromStatusCode returns the reason for the failure based on the status code provided.
func GetFailureReasonFromStatusCode(statusCode int) Reason {
	if statusCode/100 == 4 {
		return ClientErrorReason
	}
	if statusCode/100 == 5 {
		return ServerErrorReason
	}

	return DefaultReason
}

Suggested solution:

If Opsgenie can return a 4xx to Alertmanager for all alerts failing for creation, the customer can react based on the metrics from Alertmanager for those misses.

Workaround:
Read the error field in the current response payload to determine if alert creation has failed in Opsgenie.