see more from https://github.com/openzipkin/brave/issues/867, also see https://github.com/apache/incubator-dubbo/issues/3576
if (attachment != null && attachment.size() > 0) {
            this.attachments.putAll(attachment);
        }
        return this;
    }
    public void clearAttachments() {
        this.attachments.clear();
    }
    /**
     * get values.
     *
     * @return values
     */
    public Map<String, Object> get() {
        return values;
    }
    /**
     * set value.
     *
     * @param key
     * @param value
     * @return context
     */
    public RpcContext set(String key, Object value) {
        if (value == null) {
            values.remove(key);
        } else {
            values.put(key, value);
        }
        return this;