Support duration as nanosecond number when using a resolver

Duration is always marshaled as a nanosecond number when using JSON. Kong would only parse it as a string using ParseDuration. Now it can use the number version too.

mapper.go

@@ -308,15 +308,23 @@ func (boolMapper) IsBool() bool { return true }
 
 func durationDecoder() MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
-		var value string
-		if err := ctx.Scan.PopValueInto("duration", &value); err != nil {
+		t, err := ctx.Scan.PopValue("duration")
+		if err != nil {
 			return err
 		}
-		r, err := time.ParseDuration(value)
-		if err != nil {
-			return errors.Errorf("expected duration but got %q: %s", value, err)
+		var d time.Duration
+		switch v := t.Value.(type) {
+		case string:
+			d, err = time.ParseDuration(v)
+			if err != nil {
+				return errors.Errorf("expected duration but got %q: %s", v, err)
+			}
+		case int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64, float32, float64:
+			d = reflect.ValueOf(v).Convert(reflect.TypeOf(time.Duration(0))).Interface().(time.Duration) // nolint: forcetypeassert
+		default:
+			return errors.Errorf("expected duration but got %q", v)
 		}
-		target.Set(reflect.ValueOf(r))
+		target.Set(reflect.ValueOf(d))
 		return nil
 	}
 }

mapper_test.go

@@ -127,6 +127,18 @@ func TestDurationMapper(t *testing.T) {
 	require.Equal(t, time.Second*5, cli.Flag)
 }
 
+func TestDurationMapperJSONResolver(t *testing.T) {
+	var cli struct {
+		Flag time.Duration
+	}
+	resolver, err := kong.JSON(strings.NewReader(`{"flag": 5000000000}`))
+	require.NoError(t, err)
+	k := mustNew(t, &cli, kong.Resolvers(resolver))
+	_, err = k.Parse(nil)
+	require.NoError(t, err)
+	require.Equal(t, time.Second*5, cli.Flag)
+}
+
 func TestSplitEscaped(t *testing.T) {
 	require.Equal(t, []string{"a", "b"}, kong.SplitEscaped("a,b", ','))
 	require.Equal(t, []string{"a,b", "c"}, kong.SplitEscaped(`a\,b,c`, ','))