code: public LogNode log(final String sortKey, final String msg, final long elapsedTimeNanos) {
          return addChild(sortKey, msg, elapsedTimeNanos);
      }
docs: Add a log entry with sort key for deterministic ordering.
  
  @param sortKey
  The sort key for the log entry.
  @param msg
  The message.
  @param elapsedTimeNanos
  The elapsed time.
  @return a child log node, which can be used to add sub-entries.