Add cert lifetime metrics

[atc0005]

With some certificates having (at present) 398 days maximum and others having 90 days we need to provide a way to indicate (programmatically) what lifetime remains for a certificate. Said another way, a certificate with a maximum lifetime of 90 days having 30 days remaining (presumably managed by an agent) is less of a concern than a certificate with a maximum lifetime of 398 days that has 30 days remaining (presumably not managed by an agent).

One use case is collecting these metrics and using a reporting tool to provide a % listing of a cert's remaining lifetime.

It might also be useful to provide metrics for the chain as a whole (using the certificate with the lowest lifetime in the chain).

EDIT:

Perhaps per chain position lifetime metric also?

See also:

• Add support for cert lifetime percentage based thresholds #789