Alerting: Fix stale values associated with states that have gone to NoData, unify values calculation (#89807)

* Unify values

* Fix with latest changes on main

* Fix up NaN test

* Keep refIDs with -1 as value

* Test that refIDs are preserved on Normal to Error transition

* Alerting to err test too

* Add a blurb to docs about this behavior

pkg/services/ngalert/api/testing.go

@@ -86,7 +86,7 @@ func (f *fakeAlertInstanceManager) GenerateAlertInstances(orgID int64, alertRule
 			LatestResult: &state.Evaluation{
 				EvaluationTime:  evaluationTime.Add(1 * time.Minute),
 				EvaluationState: eval.Normal,
-				Values:          make(map[string]*float64),
+				Values:          make(map[string]float64),
 			},
 			LastEvaluationTime: evaluationTime.Add(1 * time.Minute),
 			EvaluationDuration: evaluationDuration,