public RetryConfigBuilder<T> toBuilder() {
        final RetryConfigBuilder<T> builder;
        if (retryRuleWithContent != null) {
            builder = builder0(retryRuleWithContent).maxContentLength(
                    maxContentLength);
        } else {
            assert retryRule != null;
            builder = builder0(retryRule);
        }
        return builder
                .maxTotalAttempts(maxTotalAttempts)
                .responseTimeoutMillisForEachAttempt(responseTimeoutMillisForEachAttempt);
    }
Converts this {@link RetryConfig} to a mutable {@link RetryConfigBuilder}.
core/src/main/java/com/linecorp/armeria/client/retry/RetryConfig.java