contrib/log/slog: implement handler.WithAttrs

[shota3506]

What does this PR do?

implement contrib/log/slog.handler.WithAttrs

Motivation

resolve #2838

Note

In the issue, I mentioned that the handler is replaced by Logger.WithGroup call as well. But I'm not sure if it is fine to implement handler.WithGroup in this case.

If you implement logger.WithGruop like

func (h *handler) WithGroup(name string) slog.Handler {
	return WrapHandler(h.Handler.WithGroup(string))
}

the logger will produce the log with grouped dd attributes.

logger := slog.New(NewJSONHandler(b, nil))
logger := logger.WithGruop("group")

{"time":"2024-08-31T13:14:16.583178+09:00","level":"INFO","msg":"this is an info log with tracing information","group":{"dd.trace_id":1371324856057376362,"dd.span_id":1371324856057376362}}

But I think it is not what datadog expected.

I would like some advice on this.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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