Implement a metrics endpoint for cos-alerter, so that we can get historical availability

[sajoupa]

Enhancement Proposal

With cos-alerter, we can only see if COSes are up or down at the moment, but not history about that (when did the outage start, how long did it last, is it reoccurring, etc.)
This should be made available to a remote COS through a relation, creating the dashboard automatically.

[dstathis]

COS Alerter is a watchdog service. The purpose is not to provide advanced monitoring capabilities. For what you are trying to achieve, I believe the best solution would be to deploy a second COS instance and have them monitor each other. This would give much more data than cos-alerter could ever provide.