Introduce metrics related to certificates

[td2501]

This PR introduces metrics related to certificates:

• count of loaded certificates
• certificate validity
• time till certificate expiration

Time till certificate expiration will be negative for expired certificates.

[laura-zelenku]

Hello @td2501, I've updated my testing environment for your change and going to process your PR. I need to update documentation and some more details. I'll probably create new PR and give it to you for review. You know, to verify it contains all the features you need and also for your own testing.

[torilabs]

Hi @td2501, I'm thinking why there is serial number in dec format and not hex format? Can we discuss what is better format? I think it is hex form as this form is displayed also in regular openssl certificate printout. What do you think?

[td2501]

Hi @td2501, I'm thinking why there is serial number in dec format and not hex format? Can we discuss what is better format? I think it is hex form as this form is displayed also in regular openssl certificate printout. What do you think?

There was no real reason to export certificate serial numbers as integers. I believe the hexadecimal format with colons is a better approach, and I will implement it at the beginning of next month, once I return from vacation.