How to use Google Cloud Datastore Statistics
You can't, that's not what it's for at all. It's only for very broad-grained statistics about the number of each types in the datastore. It'll give you a rough estimate of how many Person objects there are in total, that's all.
How can I use Google Cloud Datastore stats object (in Python ) to get  the number of entities of one kind (i.e. Person) in my database satisfying a given constraint (i.e. age>20)?