Migrate gitlab-exporter to lightweight Rack server

In &5035 we identified gitlab-exporter to consume a lot of memory considering the small amount of work it performs. We believe this is largely due to us running this service on Puma. We should consider moving this service to a more lightweight Rack server such as WEBRick.

Some considerations:

• gitlab-exporter is a Sinatra app
• We already serve metrics for .com from a Puma master via an embedded WEBRick
• We should verify how many requests we currently serve from gitlab-exporter's metrics endpoints
• We need to find out whether this change needs to be made in gitlab-exporter itself or in the deployment infra such as Omnibus or Helm

TODO:

• Add server config in gitlab-exporter: https://gitlab.com/gitlab-org/gitlab-exporter/-/merge_requests/127
• emit server config in Omnibus: omnibus-gitlab!4896 (merged)
• emit server config in GitLab chart: gitlab-org/charts/gitlab!1745 (merged)