Add end-to-end test for the replication lag message when pushing over HTTP to a Geo secondary node

Proposal

After https://gitlab.com/gitlab-org/gitlab-ee/issues/9195, we should probably add an end-to-end test for the replication lag message when pushing over HTTP to a Geo secondary node.

Because the request lifecycle is somewhat long and complex: https://gitlab.com/gitlab-org/gitlab-ee/issues/9195#note_205669545

And e.g. request specs only exercise Rails.

sequenceDiagram
  participant C as Git client
  participant N as NGINX
  participant W as Workhorse
  participant R as Rails
  participant G as Gitaly
  note over C: After redirect from secondary<br/>Request against primary node
  C->>N: GET /geo-node-referrer/2/foo/bar.git/info/refs/?service=git-receive-pack
  N->>W: 
  W->>R: 
  R-->>W: 401 Unauthorized
  W-->>N: 
  N-->>C: 
  C->>N: GET /geo-node-referrer/2/foo/bar.git/info/refs/?service=git-receive-pack
  N->>W: 
  W->>R: 
  R-->>W: Render Workhorse OK
  W-->>N: 
  N-->>C: 
  C->>N: POST /geo-node-referrer/2/foo/bar.git/git-receive-pack
  N->>W: 
  W->>R: GitHttpController#git_receive_pack
  note over R: Cache<br/>params[:geo_node_referrer_id]
  R-->>W: Render Workhorse OK
  W->>G: Workhorse gets the connection details from Rails, connects to Gitaly: SmartHTTP Service, ReceivePack RPC (check the proto for details)
  G->>R: POST /api/v4/internal/allowed
  R->>G: 200 OK
  G->>R: POST /api/v4/internal/post_receive
  note over R: Fetch Geo node referrer.<br/>And add lag message<br/>to this response.
  R->>G: 200 OK
  G-->>W: Return a stream of Proto messages
  W-->>N: Pipe these messages through, no transformation
  N-->>C: Pipe messages to the Git client

cc @jennielouie Feel free to close or modify as you wish.