[Metrics Alerts] Add OUTSIDE RANGE threshold condition

[Zacqary]

Design docs for metrics alerts included two additional comparators that we didn't implement in time for 7.7. These are:

• OUTSIDE RANGE - basically the opposite of BETWEEN, alerts when a value is not between two values
• IS EMPTY - alerts when the value is zero? @sorantis please clarify

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)

[sorantis]

My initial proposal was to let the user set IS EMPTY to alert when there are no new data points for the selected metric.

[Zacqary]

Right now we have an alert state implemented called No Data; this is distinct from an Alert state or an OK state, but as of right now it's just stored as a state value and doesn't send any messages out. The intention is that we'd eventually reveal this through the alert management/history UI.

Should we just have alerts send a message when they hit a No Data state instead of making this a distinct operator? For example, if we create an alert that's supposed to fire when a metric goes above 1.0, and then that metric suddenly stops reporting, that might indicate some kind of infrastructure failure that the user might want to know about.

[sorantis]

Ah, you mean like having an option on the Alert dialog?

• Alert me if there's no data

[Zacqary]

That could work! Should it be checked by default?

[sorantis]

I doubt. Let's keep it unchecked. This option can make a lot of sense for static resources, like hosts, but less so for dynamic resources.

The question is when should we fire such alert? I see two options -

1. Let the user worry about it and make it a parameter to configure:
   [ ] Alert me if there's no data for X minutes.
2. Give the user less parameters to worry about. Think of a good default value and use it. A hardcoded number obviously won't work, but something proportional to a check interval could do.

[Zacqary]

Looks like OUTSIDE_RANGE is fairly simple and adding the option to alert for no data is a bit more involved. I'll push two separate PRs for them

[Zacqary]

Going to branch off IS EMPTY into a second issue #64080