Alerting: Add backend support for keep_firing_for (#100750)

What is this feature?

This PR introduces a new alert rule configuration option, keep_firing_for (Prometheus documentation).

keep_firing_for prevents alerts from resolving immediately after the alert condition returns to normal. Instead, they transition into a "Recovering" state and are not considered resolved by the Alertmanager. Once the recovery period ends (or after the next evaluation if it is bigger than keep_firing_for), the alert transitions to "Normal" if it doesn't start alerting again:

Before                                          

+----------+     +----------+                    
| Alerting |---->|  Normal  |                    
+----------+     +----------+                    

-----
After

+----------+      +------------+     +----------+
| Alerting |----->| Recovering |---->|  Normal  |
+----------+      +------------+     +----------+                                                 

Why do we need this feature?

This feature prevents flapping alerts by adding a recovery period. This helps avoid false resolutions caused by brief alert

pkg/services/sqlstore/migrations/migrations.go

@@ -148,6 +148,8 @@ func (oss *OSSMigrations) AddMigration(mg *Migrator) {
 
 	ualert.AddAlertRuleGuidMigration(mg)
 
+	ualert.AddAlertRuleKeepFiringFor(mg)
+
 	ualert.AddAlertRuleMissingSeriesEvalsToResolve(mg)
 
 	ualert.AddAlertRuleVersionUIDIndex(mg)

pkg/services/sqlstore/migrations/ualert/alert_rule_keep_firing_for.go

@@ -0,0 +1,17 @@
+package ualert
+
+import "github.com/grafana/grafana/pkg/services/sqlstore/migrator"
+
+// AddAlertRuleKeepFiringFor adds keep_firing_for column to alert_rule and alert_rule_version tables.
+func AddAlertRuleKeepFiringFor(mg *migrator.Migrator) {
+	column := &migrator.Column{Name: "keep_firing_for", Type: migrator.DB_BigInt, Nullable: false, Default: "0"}
+
+	mg.AddMigration(
+		"add keep_firing_for column to alert_rule",
+		migrator.NewAddColumnMigration(migrator.Table{Name: "alert_rule"}, column),
+	)
+	mg.AddMigration(
+		"add keep_firing_for column to alert_rule_version",
+		migrator.NewAddColumnMigration(migrator.Table{Name: "alert_rule_version"}, column),
+	)
+}