Alerting: Add a "Reason" to Alert Instances to show underlying cause of state. (#49259)

This change adds a field to state.State and models.AlertInstance
that indicate the "Reason" that an instance has its current state. This
helps us account for cases where the state is "Normal" but the
underlying evaluation returned "NoData" or "Error", for example.

Fixes #42606

Signed-off-by: Joe Blubaugh <joe.blubaugh@grafana.com>

pkg/services/ngalert/api/api_prometheus.go

@@ -58,9 +58,16 @@ func (srv PrometheusSrv) RouteGetAlertStatuses(c *models.ReqContext) response.Re
 		alertResponse.Data.Alerts = append(alertResponse.Data.Alerts, &apimodels.Alert{
 			Labels:      alertState.GetLabels(labelOptions...),
 			Annotations: alertState.Annotations,
-			State:       alertState.State.String(),
-			ActiveAt:    &startsAt,
-			Value:       valString,
+
+			// TODO: or should we make this two fields? Using one field lets the
+			// frontend use the same logic for parsing text on annotations and this.
+			State: state.InstanceStateAndReason{
+				State:  alertState.State,
+				Reason: alertState.StateReason,
+			}.String(),
+
+			ActiveAt: &startsAt,
+			Value:    valString,
 		})
 	}
 
@@ -212,9 +219,16 @@ func (srv PrometheusSrv) toRuleGroup(groupName string, folder *models.Folder, ru
 			alert := &apimodels.Alert{
 				Labels:      alertState.GetLabels(labelOptions...),
 				Annotations: alertState.Annotations,
-				State:       alertState.State.String(),
-				ActiveAt:    &activeAt,
-				Value:       valString,
+
+				// TODO: or should we make this two fields? Using one field lets the
+				// frontend use the same logic for parsing text on annotations and this.
+				State: state.InstanceStateAndReason{
+					State:  alertState.State,
+					Reason: alertState.StateReason,
+				}.String(),
+
+				ActiveAt: &activeAt,
+				Value:    valString,
 			}
 
 			if alertState.LastEvaluationTime.After(newRule.LastEvaluation) {