I didn't see anything about the robots.txt approach, but this support page discusses the meta tag method. https://support.google.com/webmasters/answer/6229325?

I didn't see anything about the robots.txt approach, but this support page discusses the meta tag method. https://support.google.com/webmasters/answer/6229325?hl=en https://support.google.com/webmasters/answer/6229325?hl=en