We shouldn't access the undocumented `_fmt` attribute here.  I think we should probably manage our own private attribute for the format string since we aren't using the base class's format code.  (for datefmt I could go either way - use a private attribute and format ourselves like we always have, or pass it to the base class and start using formatTime).

Also, it should be documented that our format string behaves slightly differently from the default Formatter implementation: the message is always appended at the end, and `%(message)s` should not be included in the format string.