log: expose the InstanceID as per the python API's InstanceKey.
log: expose the InstanceID as per the python API's InstanceKey.

Change-Id: I1c3545d57a53d9c690b55043097620f75d9d4058

flowlo/appengine,skyeunknown084/appengine,skyeunknown084/appengine,mzimmerman/appengine,golang/appengine,mzimmerman/appengine,golang/appengine,flowlo/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,skyeunknown084/appengine,mzimmerman/appengine,flowlo/appengine,fuchsia-mirror/third_party-github.com-golang-appengine