cert expiration monitoring

[aieri]

Enhancement Proposal

As traefik can handle TLS termination for multiple services, it would be the perfect place for monitoring certificate expiration. This could for example be a metric indicating the time left until expiration, plus an alert rule firing 30 days prior to it.

Note: as an MSP, a (legacy) equivalent setup - part of openstack-service-checks - has been crucial for us in order to get the ball rolling in time for certificate renewals in Charmed OpenStack clouds. Traefik could do something similar for any service that relies on it (and be well integrated with COS).