Expose target TLS cert expiration in api-server Prometheus endpoint

[XioNoX]

A bit similar to #120

To not have to rely on an additional monitoring tool, it would be useful that gNMIc exposes the targets TLS certificate expiration date on its Prometheus endpoint in order to alert when one is about to expire.

Something like: gnmic_target_certificate_expiration{target="my.device.fqdn:32767"} 1727078377

Thanks!