Alerting: Keep last state on no data (#6354)

* feat(alerting): add support to keep last state on no data

closes #6332

* refactoring(alerting PR #6354): added new option type for NoData option so AlertStateType does not have to contain invalid state, #6354

pkg/models/alert.go

@@ -8,6 +8,7 @@ import (
 
 type AlertStateType string
 type AlertSeverityType string
+type NoDataOption string
 
 const (
 	AlertStateNoData    AlertStateType = "no_data"
@@ -17,10 +18,25 @@ const (
 	AlertStateOK        AlertStateType = "ok"
 )
 
+const (
+	NoDataSetNoData   NoDataOption = "no_data"
+	NoDataSetAlerting NoDataOption = "alerting"
+	NoDataSetOK       NoDataOption = "ok"
+	NoDataKeepState   NoDataOption = "keep_state"
+)
+
 func (s AlertStateType) IsValid() bool {
 	return s == AlertStateOK || s == AlertStateNoData || s == AlertStateExecError || s == AlertStatePaused
 }
 
+func (s NoDataOption) IsValid() bool {
+	return s == NoDataSetNoData || s == NoDataSetAlerting || s == NoDataSetOK || s == NoDataKeepState
+}
+
+func (s NoDataOption) ToAlertState() AlertStateType {
+	return AlertStateType(s)
+}
+
 type Alert struct {
 	Id             int64
 	Version        int64