Add `min` expiration lifetime value to `expires_leaf`, `expires_intermediate` performance data metrics

[atc0005]

Overview

As part of the work for GH-465, support for emitting multiple performance data metrics was added.

These two in particular:

• expires_leaf
• expires_intermediate

indicate:

• label
• current value
• warning threshold
• critical threshold

Example metrics emitted:

'expires_intermediate'=611d;60;30;; 'expires_leaf'=24d;60;30;;

Two fields are not populated:

• min
• max

We don't know the value to emit for max, so we leave that field blank. If we continue to use 0 as our baseline (not reporting expired certificates with a negative value), then we do know the minimum value supported: 0.

We can populate the min field so that graphing tools will have that value available.

References

• GH-465