I use google managed-VMs. My module has several instances. I need to perform a HTTP call on each of these instances. Hence, I would like to know if there is a way to specify the recipient instance in the URL.
For instance I tried: http://<instance-name>.<module-name>.<project-name>.appspot.com/ without success.
You're missing a <version-name> in there:
http://<instance-name>.<version-name>.<module-name>.<project-name>.appspot.com
See https://cloud.google.com/appengine/docs/python/modules/#Python_Application_hierarchy for more details (this is the page for Python, but it applies across-the-board.