private String mergeMessage(String msg, StringBuilder sb) {
        if(sb != null) {
            sb.append(msg);
            return sb.toString();
        } else {
            return msg;
        }
    }
Prepend markers and key-value pairs to the message.

@param aLoggingEvent

@return
slf4j-api/src/main/java/org/slf4j/spi/DefaultLoggingEventBuilder.java