Make gitlab-metrics-exporter available to gitlab-rails

Once we have a working project pipeline via #355433 (closed), we need to define a mechanism for how to publish build artifacts. Eventually the system should be able to run in two different modes:

• As a sidecar process to gitlab-rails (this issue).
• In its own container in any CNG setup (#355449 (closed)).

In "sidecar mode", the exporter's life-cycle is handled by the Rails app. It is simply a binary we shell out to. It therefore must be installed into the webservice and sidekiq containers. Since in this mode the exporter must share a filesystem with Rails, it should probably not run in a separate container. We should work with groupdistribution to define how to best approach this.