[BUG] contrib/log/slog: log/slog.Handler is replaced by internal handler after Logger.With or Logger.WithGroup call

[shota3506]

Version of dd-trace-go

v1.67.0

Describe what happened:
dd-trace-go.v1/contrib/log/slog.Handler is replaced by internal handler after Logger.With or Logger.WithGroup call.

That's because it doesn't implement its own WithAttrs and WithGroup and just uses internal handler's method.
https://cs.opensource.google/go/go/+/refs/tags/go1.23.0:src/log/slog/logger.go;l=132

Describe what you expected:

slog.Logger keeps dd-trace-go.v1/contrib/log/slog.Handler after Logger.With or Logger.WithGroup call.

Steps to reproduce the issue:

You can confirm that the logger's Handler is replaced after logger.With or logger.WithGroup call.

package main

import (
	"fmt"
	"log/slog"
	"os"

	ddslog "gopkg.in/DataDog/dd-trace-go.v1/contrib/log/slog"
)

func main() {
	logger := slog.New(ddslog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{}))

	fmt.Printf("before logger.With call: %T\n", logger.Handler())
	// before With call: *slog.handler

	logger = logger.With("key", "value")

	fmt.Printf("after logger.With call: %T\n", logger.Handler())
	// after With call: *slog.JSONHandler
}

Additional environment details (Version of Go, Operating System, etc.):

N/A

[shota3506]

I addressed the issue in this #2839

As I mentioned in the PR comment, I'm not sure how I should treat Logger.WithGroup call in this case.
I would like some advice on this.

[darccio]

@shota3506 Thanks for reaching out. We've reviewed it and we understand your concern. We'll check it soon.

[shota3506]

It might be a possible solution that the handler holds attributes and groups as its private fields and passes them to the inner handler at handler.Handle method. Although it will make the handler implementation complex and a little bit slow.
https://cs.opensource.google/go/x/exp/+/e7e105de:slog/handler.go;l=223-253

I thought ReplaceAttr resolves this issue, but it cannot modify the attribute's group as far as I checked
https://pkg.go.dev/golang.org/x/exp/slog#HandlerOptions