Clarify healthz `unhealthy-count` and `last-unhealthy`

[hellt]

Hi @robshakir @marcushines

Can we clarify the difference in descriptions for last-unhealthy and unhealthy-count?
Namely, last-unhealthy indicates that the field should be modified any time a component is in a status that is not HEALTHY. UNSPECIFIED falls under this category.

At the same time, unhealthy-count only counts transitions HEALTHY<->UNHEALTHY. Will unhealthy counter increment if the component moved from HEALTHY to UNSPECIFIED?

This subtle difference may lead to differences in implementation.

public/release/models/platform/openconfig-platform-healthz.yang

Lines 78 to 122 in de94e3b

	leaf status {
	type enumeration {
	enum UNSPECIFIED {
	description
	"The component's health status has not yet been checked
	by the system.";
	}
	enum HEALTHY {
	description
	"The component is in a HEALTHY state, and is operating
	within the expected parameters.";
	}
	enum UNHEALTHY {
	description
	"The component is in a unhealthy state, it is not
	performing the function expected of it.";
	}
	}
	description
	"The status of the component, indicating its current health.";
	oc-ext:telemetry-on-change;
	}
	leaf last-unhealthy {
	type oc-types:timeticks64;
	description
	"The time at which the component as last observed to be unhealthy
	represented as nanoseconds since the Unix epoch. Unhealthy is defined
	as the component being in a state other than HEALTHY.";
	oc-ext:telemetry-on-change;
	}
	leaf unhealthy-count {
	type uint64;
	description
	"The number of status checks that have determined this component
	to be in an unhealthy state. This counter should be incremented
	when the component transitions from the HEALTHY to UNHEALTHY
	state such that the value reflects the number of times the
	component has become unhealthy.";
	oc-ext:telemetry-on-change;
	}
	}

To summarize, our reading of current model digests to the following state transition:

1. healthy->unspecified | last-unhealthy changes | unhealthy-count doesn't change
2. unhealthy->unspecified | last-unhealthy changes | unhealthy-count doesn't change
3. unspecified->unhealthy | last-unhealthy changes | unhealthy-count doesn't change
4. unspecified->healthy | last-unhealthy doesn't change | unhealthy-count doesn't change
5. healthy->unhealthy | last-unhealthy changes | unhealthy-count increments change
6. unhealthy->healthy | last-unhealthy doesn't change | unhealthy-count doesn't change

[marcushines]

it should be once healthy it is any state transition out of that state

[hellt]

@marcushines then this addition should be valid - #853. If I understood your comment correctly.

[dplore]

Fixed by #853