add dataset_status='normal|blocked' to job metrics
It will help show more precise graphs in Grafana, separating the jobs for blocked datasets from the rest of the jobs.

Two notes:
- the lookup for jobs in blocked datasets is maybe not optimal
- we only take this into account in the cron job that recomputes the metrics, not in every job creation/update/deletion (ie: every 10 minutes). I think it's good enough
add dataset_status='normal|blocked' to job metrics: It will help show more precise graphs in Grafana, separating the jobs for blocked datasets from the rest of the jobs.

Two notes:
- the lookup for jobs in blocked datasets is maybe not optimal
- we only take this into account in the cron job that recomputes the metrics, not in every job creation/update/deletion (ie: every 10 minutes). I think it's good enough