Limit metrics (of kubeconfig-rotation) collected only to those that are essential

[Disper]

Description
Some of the collected metrics are or might not be essential:

• expires (timestamp) label (as it is redundant because it's stored also as value)
• rotate after (seconds) - align with SRE if this property is needed
• expires (seconds)- align with SRE if this property is needed

Reasons

Having too many metrics might greatly increase the cost of storing the metrics.

Attachments

• https://www.robustperception.io/cardinality-is-key/

[Disper]

While working on this issue, consider if something from #180 could also be done.

[atisap]

• Removing the label, called expires will decrease the churn rate, I would suggest that.
• expires (seconds) which is the expirationDuration label could useful, because it is different on dev stage and prod environment, and we could check with that label if the expiration time reached the expirationDuration time. A good indicator to check if the kubeconfig not expired in time, probably because not rotated.
• Regarding the rotate after (seconds) which is the rotationDuration label, an alert can be build to check if the expires time went beyond the rotationDuration, which mean the rotation not happened in time.
  As I see, the rotationDuration is the 40% of the expirationDuration on all environments. During building an alert, this result can reach without the rotationDuration by a simple calculation. The alerts are in progress, so can't state that this label not needed right now, but I will update the ticket.

[tobiscr]

Relates as well to #113