Resolves: #15559
As accurately noted in the issue description, the map is shared among
child loggers that get created when `WithAttr()`/`WithGroup()` are
called on the underlying handler, which happens via `log.With()` and
`log.WithGroup()` respectively.
The RW mutex was a value in the previous implementation that used
go-kit/log, and I should've updated it to use a pointer when I converted
the deduper.
Also adds a test.
Signed-off-by: TJ Hoplock <t.hoplock@gmail.com>
437362a7a7