Allow to specify which server to use

Refs gitlab-org/gitlab#293687 (closed)

We currently let Sinatra auto-pick a Rack handler based on the gems that are loaded.

This happens to imply that gitlab-exporter will run on Puma. Puma is fast, but consumes more memory than WEBRick. We think that we should default to a more resource friendly alternative. The traffic served from gitlab-exporter is very low; it is only contacted by the Prometheus scraper every 10 seconds or so.

However, I kept this configurable by adding a new settings key.