[Alerting] JIRA connector to append comments to existing tickets

[afharo]

Describe the feature:
From https://discuss.elastic.co/t/alerts-with-the-combination-of-multiple-metrics/247653

When I attach a jira action to the alert, when the alert condition is met, the jira is being created even if the jira was already created in the previous check.
Is it possible to add the comment if the jira is already present?

[elasticmachine]

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

[pmuellr]

I'm not completely familiar with the Jira connector, but the main problem with this kind of workflow is that the alert has to "remember" the id of the Jira ticket that was originally created. But we don't really have a way of doing that today. We don't really process the output of action execution in any meaningful way, and make it available back to the alert.

We have some ways of doing that - alert state and instance state - but this state is "forgotten" if the alert is ever disabled. And in general seems a bit too fragile to me.

This sounds like a scenario that the "cases" integration should probably handle, which provides a more complete life cycle story for incident management systems like this.