Rename gitlab-monitor to gitlab-exporter

As part of https://gitlab.com/gitlab-org/gitlab-ce/issues/61704 we want to rename the project, to be less confusing to users and conform to naming conventions for Prometheus exporters.

This PR renames:

• The gem from gitlab-monitor to gitlab-exporter
• The executable from gitlab-mon to gitlab-exporter
• The ruby module from Gitlab::Monitor to Gitlab::Exporter
• The config from config/gitlab-monitor*.yml to config/gitlab-exporter*.yml

At some point the repo has to be renamed too, for now I'm unsure of the consequences