Logic to emit metrics for certificate expiration times, to create alerts and dashboards for observability and monitoring

[steffen-karlsson]

This PR provides logic to emit metrics for certificate expiration time in ms.
This is for creating alerts and dashboards for observability and monitoring.

Example of a panel in a dashboard in Grafana:

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[steffen-karlsson]

Examples of the metrics from JMX exporter:

[steffen-karlsson]

Hello @rayokota
I see that you're a quite frequent committer to this repo, I'd like to hear if possible, your feedback on this suggestion :)
Its at least something we're very interested in, in our company for observability and alerting and currently are having a not so nice solution to cater for it.

Thanks,

[rayokota]

Thanks for the PR @steffen-karlsson , LGTM