Google does not have an API to my knowledge, plus their own tool and documentation sometimes contradict (speaking from experience with TSF). If someone knows about something, it will be https://twitter.com/danbri he works for google and is author of schema.org. I'm not sure if you have twitter, I can ask him for you if you want. Alternatively https://twitter.com/regroup there are commercial solutions, however, like https://microlink.io.