Previously receivers were only validated before saving the alertmanager
configuration. This is a suboptimal experience for those upgrading with preview
as the failed channel upgrade will return an API error instead of being
summarized in the table.
Adds a feature flag (alertingUpgradeDryrunOnStart) that will dry-run the legacy
alert upgrade on startup. It is enabled by default.
When on legacy alerting, this feature flag will log the results of the legacy
alerting upgrade on startup and draw attention to anything in the current legacy
alerting configuration that will cause issues when the upgrade is eventually
performed. It acts as a log warning for those where action is required before
upgrading to Grafana v11 where legacy alerting will be removed.
This PR has two steps that together create a functional dry-run capability for the migration.
By enabling the feature flag alertingPreviewUpgrade when on legacy alerting it will:
a. Allow all Grafana Alerting background services except for the scheduler to start (multiorg alertmanager, state manager, routes, …).
b. Allow the UI to show Grafana Alerting pages alongside legacy ones (with appropriate in-app warnings that UA is not actually running).
c. Show a new “Alerting Upgrade” page and register associated /api/v1/upgrade endpoints that will allow the user to upgrade their organization live without restart and present a summary of the upgrade in a table.
Some refactoring that will simplify next changes for dry-run PRs. This should be no-op as far as the created ngalert resources and database state, though it does change some logs.
The key change here is to modify migrateOrg to return pairs of legacy struct + ngalert struct instead of actually persisting the alerts and alertmanager config. This will allow us to capture error information during dry-run migration.
It also moves most persistence-related operations such as title deduplication and folder creation to the right before we persist. This will simplify eventual partial migrations (individual alerts, dashboards, channels, ...).
Additionally it changes channel code to deal with PostableGrafanaReceiver instead of PostableApiReceiver (integration instead of contact point).
