What would you like to be added:
Enhance the cert-manager metrics to get more information of the certificate, such as the certificate expiration date.

Why is this needed:
There are several metrics provided: https://github.com/gardener/cert-management#metrics.
If want to monitor the certificate expiration information, only "cert_management_overdue_renewal_certificates" is related, it shows "Number of certificate objects with certificate's renewal overdue". But the metrics is only returned the number of overdue certificate instead of the expiration date, we can monitor the expiration date to know when the certificates will be expired.