Generate alerts rules for missing metrics for each unit

[err404r]

Bug Description

For now COS can't detect the situation when there are no metrics are coming from grafana-agent. Ideally each registered grafana agent should generate one alert rule. And alert when metrics are missing

To Reproduce

Deploy grafana-agent, relate it to cos and break grafana-agent config.
In real life it could happened when tls_insecure_skip_verify=true is not set when it is required

Environment

grafana-agent revision 134

Relevant log output

No related logs

Additional context

No response

[simskij]

Some kind of up and absent type of rule needed.