Add custom header to request

[ainsleyclark]

Is there an idiomatic way to add a header to any given request? It seems below doesn't quite work.
No header is attached to the ouath call:

type CustomTransport struct {
	Base http.RoundTripper
}

func (t *CustomTransport) RoundTrip(req *http.Request) (*http.Response, error) {
	req.Header.Add("My-Header", "whatever")
	fmt.Println(req.Header)
	return t.Base.RoundTrip(req)
}

func Temp() error {
	credentials, err := google.CredentialsFromJSON(context.Background(), []byte("my-creds"), sheets.SpreadsheetsReadonlyScope)
	if err != nil {
		return err
	}

	client := &http.Client{
		Transport: &CustomTransport{
			Base: oauth2.NewClient(context.Background(), credentials.TokenSource).Transport,
		},
	}

	sheetClient, err := sheets.NewService(context.Background(), option.WithHTTPClient(client))
	if err != nil {
		return err
	}

	do, err := sheetClient.Spreadsheets.Get("ID").Do()
	if err != nil {
		return err
	}

	fmt.Println(do)
	return nil
}