[RAC][Metrics UI] Format reason in inventory metric executor

[mgiota]

After merging #105706 inventory rule types started appearing in the Observability Alerts table. At that point we displayed a simple message in the Reason column Inventory alert for {node}. We should now index the reason field and not construct the message in the front end.

Acceptance criteria

• The inventory_metric_threshold_executor should index the reason when using the alertInstanceFactory
• inventory_metric_threshold_executor already builds the reason, so we can use that one
• Instead of constructing the reason in the front end https://github.com/elastic/kibana/blob/master/x-pack/plugins/infra/public/alerting/inventory/rule_data_formatters.ts we should retrieve it from the index.
• Create an inventory rule type with multiple conditions and make sure that the long message appears nice in the Observability alerts table

You should add following configuration to be able to see the alerts:

xpack.observability.unsafe.alertingExperience.enabled: true
xpack.ruleRegistry.write.enabled: true

[elasticmachine]

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