fix: expose runner_type directly

Before we exposed 'is_shared', but this value doesn't exist anymore since gitlab-org/gitlab-foss@ca93faf1 so it only works for previously added shared runners

I've decided to directly expose the runner_type variable, since it allows for more detailed metrics, instead of just using is_shared

Closes https://gitlab.com/gitlab-org/gitlab-exporter/issues/48

🛠 with ❤ at Siemens