Consider a "group" construct to match slog

[thockin]

#196 (comment)

Consider:

s := logr.ToSlog(myLogger)
s.With("a", 1)  // calls Handler.WithAttrs(), which calls logr WithValues("a", 1)
s = s.WithGroup("g") // calls Handler.WithGroup("g"), which saves "g" as a prefix
s.With("b", 2)  // calls Handler.WithAttrs(), which calls logr WithValues("g.b", 2)
s.Info("the msg", "c", 3) // calls Handler.Handle, which calls logr.Info("the msg", "g.c", 3)

Unless the LogSink is very clever (parse all keys and re-group, which I would not suggest doing), the result will be something like {"msg": "the msg", "a": 1, "g.b": 2, "g.c": 3}, when what the user clearly wanted was more like {"msg": "the msg", "a": 1, "g": {"b": 2, "c": 3}}

If we teach logr to have a "group" construct, maybe we can make it work:

s := logr.ToSlog(myLogger)
s.With("a", 1)  // calls Handler.WithAttrs(), which calls logr WithValues("a", 1)
s = s.WithGroup("g") // calls Handler.WithGroup("g"), which calls logr WithGroup("g")
s.With("b", 2)  // calls Handler.WithAttrs(), which calls logr WithValues("b", 2)
s.Info("the msg", "c", 3) // calls Handler.Handle, which calls logr.Info("the msg", "c", 3)