Google's App Engine is a service which lets developers create web applications without having to worry too much about server infrastructure or scaling issues.
These kinds of user agents indicate that the requests are coming from an app running on Google's App Engine.
The first part of the URL is always the same, but to differentiate the various apps which run on AppEngine there is always an App ID section at the end of the user agent. This will be different, depending on which application is accessing your site.
Sometimes, you can learn more about a particular Google AppEngine App by visiting it's unique site url, based on the app name. For example, if the appid was "sample-app", you could try visiting "sample-app.appspot.com" for more info. This doesn't seem to work for all apps however.
For more information on Google App Engine please look at Google's site: cloud.google.com/appengine