public Date retryAfter() {
        return Optional.ofNullable(retryAfter).map(Date::new).orElse(null);
    }
Sometimes corresponds to the retryAfter header present in {@code 503}
status. Other times parsed from an application-specific response. Null if unknown.

@return {@linkplain Date}
shenyu-sdk/shenyu-sdk-core/src/main/java/org/apache/shenyu/sdk/core/retry/RetryableException.java