code: private LogNode addChild(final String sortKey, final String msg, final long elapsedTimeNanos) {
          return addChild(sortKey, msg, elapsedTimeNanos, null);
      }
docs: Add a child log node for a message.
  
  @param sortKey
  the sort key
  @param msg
  the log message
  @param elapsedTimeNanos
  the elapsed time in nanos
  @return the log node