log/slog: fix string representation of Group values

Format Group values like a []Attr, rather than a *Attr.

Also, use fmt.Append in Value.append.

Updates golang#56345.

Change-Id: I9db1a8ec47f8e99c1ac3225d78e152013116bff3
Reviewed-on: https://go-review.googlesource.com/c/go/+/479515
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/log/slog/value.go

@@ -414,8 +414,10 @@ func (v Value) append(dst []byte) []byte {
 		return append(dst, v.duration().String()...)
 	case KindTime:
 		return append(dst, v.time().String()...)
-	case KindAny, KindGroup, KindLogValuer:
-		return append(dst, fmt.Sprint(v.any)...)
+	case KindGroup:
+		return fmt.Append(dst, v.group())
+	case KindAny, KindLogValuer:
+		return fmt.Append(dst, v.any)
 	default:
 		panic(fmt.Sprintf("bad kind: %s", v.Kind()))
 	}

src/log/slog/value_test.go

@@ -59,6 +59,7 @@ func TestValueString(t *testing.T) {
 		{StringValue("foo"), "foo"},
 		{TimeValue(testTime), "2000-01-02 03:04:05 +0000 UTC"},
 		{AnyValue(time.Duration(3 * time.Second)), "3s"},
+		{GroupValue(Int("a", 1), Bool("b", true)), "[a=1 b=true]"},
 	} {
 		if got := test.v.String(); got != test.want {
 			t.Errorf("%#v:\ngot  %q\nwant %q", test.v, got, test.want)