[Alerting] [o11y] Determine where to store the index

[chrisronline]

Relates to #98902 (comment)

Blocked: This is currently blocked for the same reasons as #100678

Once we determine the set of metrics that are valuable (see #100675), we want to figure out where the data should live. The obvious first choice is the same cluster which Kibana is talking to, but it might be hard to query the data if the cluster is overwhelmed. The Stack Monitoring solution already has the ability to index data into separate, dedicated cluster for monitoring storage which might be a better long term fit.

[elasticmachine]

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

[chrisronline]

Closing as the new proposal is taking down a different path in the short term