Is there a way to get the number of GAE instances that are up in real time?

Google App Engine Admin API
https://cloud.google.com/appengine/docs/admin-api/

I can get a list of instances that are started in the API below, so I think I should count them.
https://cloud.google.com/appengine/docs/admin-api/reference/rest/v1/apps.services.versions.instances/list
However, you must specify Service and Version.

I don't know why I want to know the number of instances, so I don't know the best solution, but there are several ways.