Fixes and documentation for session routing in GDK

The following discussion from gitlab-org/cells/http-router!233 (merged) should be addressed:

• @tkuah started a discussion: (+5 comments)

  Testing

  Try if it works with GDK

  Testing this is quite difficult. Lots of things to setup.

  TODO: Document this.

  1. Update ../gitlab-topology-service/config.toml to use the cell-1's address

     address = "gdk.test:12001"
     session_prefix = "cell-1"

  2. Update ../gitlab-cells/cell-1/gdk.yml to disable gitlab_topology_service, and also to set a session prefix to match:

     gitlab:
       rails:
        session_store:
           session_cookie_token_prefix: cell-1
     gitlab_http_router:
       enabled: false
     gitlab_topology_service:
       enabled: false

  3. gdk restart gitlab-topology-service
  4. gdk cells cell-1 reconfigure
  5. gdk cells cell-1 restart
  6. cd ../gitlab-http-router, and then switch to this branch
  7. cd -
  8. Edit Procfile to add --var GITLAB_RULES_CONFIG:session_prefix to the gitlab-http-router line.
  9. gdk restart gitlab-http-router
  10. Run gdk tail gitlab-http-router to show logs
  11. Add a force-cell cookie with value cell-1. This should make the router start to classify cell-1, and then route to gdk.test:12001 instead.
  12. Login using a username and password that can log into cell-1.
  13. Delete the force-cell cookie. We should still be routed to gdk.test:12001.
  14. (Optional) Set unique_cookie_key_postfix to false

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TODO: Document/FIX GDK