Create alert(s) for the case where legacy scraper targets are missing #491
Description
There was a fairly serious bug in the generate_prometheus_targets.sh file in the operator repo which resulted in all prometheus targets for legacy scraper things (e.g., sidestream.json, ssh806.json, etc.) not being generated. Yet we never knew that those targets weren't being scraped, and that they didn't exist. We should set up some absent() alerts for those (and maybe other) targets.