Add Certificate Expiration Warnings (Not Errors)

[smartinov]

Some kind of mechanism which would allow notifications of expiry on certain websites

[ppartarr]

@smartinov this has been added, the certificate expiry check happens here

@dreadl0ck this should probably be closed?

[dreadl0ck]

This is not fully implemented yet: Certificate expiry alerts are currently tracked as Errors and not as the issue title suggests as Warnings.

We do currently not have a concept of Warnings in petze,
but it would be nice to have that to describe issues that are not service failures (=Errors).

Things that produce a warning might not be critical, but still require attention (e.g: TLS cert expiry in X hours),
and could be reported in a different interval or to different contact persons.

I leave this ticket open for further brainstorming.