Allow to have a unique `user-id` across sessions (#1023)
Allow to have a unique `user-id` across sessions (#1023)

* ENABLE_UNIQUE_USER_ID_ACROSS_SESSIONS

* Update middleware.go

* fix ENABLE_UNIQUE_USER_ID_ACROSS_SESSIONS test with the value "true"

* ENABLE_SINGLE_SHARED_SESSION
GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo,GoogleCloudPlatform/microservices-demo