## Changes
It wasn't working because it deferred to the regular `slog.TextHandler`
for the `WithAttr` and `WithGroup` functions. Both of these functions
don't mutate the handler but return a new one. When the top-level logger
called one of these, log records in that context used the standard
handler instead of ours.
To implement tracking of attributes and groups, I followed the guide at
https://github.com/golang/example/blob/master/slog-handler-guide/README.md
for writing custom handlers.
## Tests
The new tests demonstrate formatting through `t.Log` and look good.
Pieter Noordhuis