protected void writeFooter() {
        if (this.out != null) {
            try {
                this.out.flush();
            }
            catch (final IOException e) {
                LogLog.error("Failed to flush writer,", e);
            }
        }
    }
Write a footer as produced by the embedded layout's
{@link Layout#getFooter} method.
metamorphosis-client-extension/src/main/java/com/taobao/metamorphosis/client/extension/log4j/StreamAppender.java