Add the ability to see when a resource was in a particular state

[ddutt]

It'd be useful to know when a resource was last in a particular state. For example, when did the MTU of the interface change last? or when was the last time the status of a port in errDisabled state, and so on.

Given that the CLI/API follow the model of , how about the use of the verb "when" for querying this behavior. For example, interface when ifname=Ethernet1/1 state=errDisabled would mean tell me the last time the interface state was in errDisabled state.

A related issue is to list all the times the value of a field changed in a given time window.

Besides general support for the idea from the users on the Slack discussion, one very interesting comment was:

What about something like a way to show the change over time… IE: “Show me the log of when this field changed?” So you wouldn’t have to know exactly what MTU you’re looking for, but you could just ask “show me every time the MTU has changed, ignore all the other changes” To be honest, we might actually look for changes in the description :laughing: - we encode some operational parameters into the descriptions: We have a [NO-MONITOR] tag to tell monitoring to not alert on something, a [PENDING] ticket_num when we don’t want to alert on a circuit just yet. If an engineer forgets to change something, a script will come around and nag them if the ticket is closed and the pending marker is still there.