datastore: unable to set query batch size

[delphinus]

I did query such below and trace of the request says there are too many datastore.next() calls.

// I used goon in real. here is equivalent code, maybe
q := datastore.NewQuery("SomeKind").KeysOnly()
for t := q.Run(ctx);; {
  k, err := t.Next(nil)

  ...

}

Issue	Description	Recommendation
Many datastore.next() calls.	Your app made 193 remote procedure calls to datastore.next() while processing this request. This was likely due the use of 20 as query batch size.	Increase the value of query batch size to reduce the number of datastore.next() calls

I cannot find the method to choose "query batch size". In NDB, batch_size option exists in query class, but GAE/Go does not seem to have.

I tried to patch query.go and set the Count (= batch_size?) to 200. I found it was finely fetching 200 entities in each Next request with no warnings.

I want to enable to set the Count with the NewQuery method. But is this a correct way?