Monitor must output 0 value for runners that are configured but have no running/pending tasks.
Following @bjk-gitlab comment on issue #48 (closed) that I had created.
A runner who has run a job (gitlab_runner_jobs > 0 at one point) will have unexploitable metrics when it is no longer metrics.
Currently, gitlab_runner_jobs is either null/absent or superior to 0.
While the metric could be usable as a scalar on a grafana graph configured to have "null as 0", it makes the metric unsable for a gauge.