Alerting: new min_interval_seconds options to enforce a minimum eval frequency (#21188)

* add min_interval_seconds setting to alerting config

It will let operator enforce a minimum time for the scheduler to enqueue evaluations

* Introduce UI modifications

* Update docs

Co-authored-by: Martin <uepoch@users.noreply.github.com>

pkg/setting/setting.go

@@ -201,6 +201,7 @@ var (
 	AlertingEvaluationTimeout   time.Duration
 	AlertingNotificationTimeout time.Duration
 	AlertingMaxAttempts         int
+	AlertingMinInterval         int64
 
 	// Explore UI
 	ExploreEnabled bool
@@ -961,6 +962,7 @@ func (cfg *Cfg) Load(args *CommandLineArgs) error {
 	notificationTimeoutSeconds := alerting.Key("notification_timeout_seconds").MustInt64(30)
 	AlertingNotificationTimeout = time.Second * time.Duration(notificationTimeoutSeconds)
 	AlertingMaxAttempts = alerting.Key("max_attempts").MustInt(3)
+	AlertingMinInterval = alerting.Key("min_interval_seconds").MustInt64(1)
 
 	explore := iniFile.Section("explore")
 	ExploreEnabled = explore.Key("enabled").MustBool(true)