Add support for parsing duration (time.ParseDuration) on custom resource state metrics

[solidDoWant]

What would you like to be added:

I would like custom state metrics to be able to parse resource duration fields into metric value. For example, they should be able to parse duration: 1h into 3600.

Why is this needed:

Some resources (like cert-manager certificates) store valuable information, such as how long a certificate is valid for, as a duration string.

Describe the solution you'd like

I would like KSM to support parsing these values, similarly to other values, as described here.

Additional context

This is not as trivial as adding a three line if this can be parsed; set the metric to the time.ParseDuration value. Custom resources already support parsing resource quantities such as 250m (i.e. 1/4 of a CPU), which is indistinguishable from the duration 250m (i.e. four hours and ten minutes). The config likely would need to support explicitly stating how a metric should be converted (which is probably desirable anyway).

[richabanker]

/assign @rexagod
/triage accepted