`SetAttrs` does not handle concurrent writes

While potentially unlikely, calling [SetAttrs](https://github.com/go-chi/httplog/blob/master/context.go#L19) could occur simultaneously from multiple go-routines, however access to the underlying slice of `[]slog.Attr` is not protected from concurrent access.

At the very least I believe this should be mentioned in the documentation but likely should be addressed to ensure data integrity.

Happy to submit a PR if there is an appetite to address this.