This commit adds a number of new metrics:

* ci_runners_active: The number of active runners
* ci_runners_instance_type_active: The number of active instance runners
* ci_runners_group_type_active: The number of active group runners
* ci_runners_project_type_active: The number of active project runners
* ci_runners_instance_type_active_online: The number of active and
  online instance runners
* ci_runners_group_type_active_online: The number of active and online
  group runners
* ci_runners_project_type_active_online: The number of active and online
  project runners

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/27141

Changelog: added