Add "only once" notificiation frequency

[thomasneirynck]

Describe the feature:

Add an "only once" or "at most once" notification option to the alertring framework. When this is selected, only a single notification will be sent as long as the alert is activated.

This would avoid duplicate alerts.

Describe a specific use case for the feature:

This is useful to notify based on "transitions", rather than an evaluation of "current state". When the alert activates, a notification is sent to signal the transition.

This would be particularly useful for the "containment"-option of geo-thresholds. #80749

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[mikecote]

Hi @thomasneirynck, the request sounds similar to Ability to throttle alert instances until state changes #50077? If so, we can close this one as we're tracking the feature under that issue and are planning to work on it in 7.11 🙂

[thomasneirynck]

@mikecote yup, sounds the same, thx for flagging!