discovery engine : how to get model scores in search result

[SherryDog]

I built a search app with discovery engine (Gen AI), and want to get the model scores. According to the doc,  

https://cloud.google.com/discovery-engine/docs/reference/rpc/google.cloud.discoveryengine.v1beta#sea...

model_scores could be available, but I did not figure out a way (either in REST API or Python sdk) to retrieve it in SearchResponse. 

Any suggestion? thanks