public OutlierDetectingRuleBuilder onServerError() {
        return onResponseHeaders((ctx, headers) -> {
            if (headers.status().isServerError()) {
                return OutlierDetectionDecision.FAILURE;
            }
            return OutlierDetectionDecision.NEXT;
        });
    }
Reports a failure if the response status is a server error (5xx).
core/src/main/java/com/linecorp/armeria/common/outlier/OutlierDetectingRuleBuilder.java