final RetryConfigMapping<O> mapping() {
        if (mapping == null) {
            final RetryConfig<O> config = retryConfig();
            assert config != null;
            return (ctx, req) -> config;
        }
        return mapping;
    }
Creates a new builder with the specified {@link RetryConfigMapping}.
core/src/main/java/com/linecorp/armeria/client/retry/AbstractRetryingClientBuilder.java