Cells: Introduce Topology Service configuration

Overview

The general information can be found in Cells: SSH Routing for Git Pull and Push (&13532)

Topology Service is a gRPC server that provides an answer to the following question: To which cell an HTTP request related to a particular record (project, group, etc...) should be routed?

Requirement

Introduce Topology Service configuration on GitLab Shell.

This commit !969 (c4a952fc) partially does it. It reads for the following entries in the config.yml file:

cells:
  url: ...

We can make it more explicit:

cells:
  global_service_url: ...

This issue is about making this functionality production-ready. But it's currently blocked by Topology Service production readiness.

Documentation

Information about classify service should be available here:

• https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/cells/topology_service/#classify-service
• https://gitlab.com/gitlab-org/cells/topology-service/-/blob/0bb2a1ad49006c94bf7ba9e6096734b9c0b01b02/proto/classify_service.proto

source