SSE/Alerting: Support prom instant vector responses (#44865)

* SSE/Alerting: (Draft) Support prom instant vector responses
fixes #35663
* reduce\classic expressions to handle mathexp.Number
* use Notice for warning

Co-authored-by: Yuriy Tseretyan <yuriy.tseretyan@grafana.com>

pkg/expr/mathexp/reduce.go

@@ -100,6 +100,11 @@ func GetReduceFunc(rFunc string) (ReducerFunc, error) {
 	}
 }
 
+// GetSupportedReduceFuncs returns collection of supported function names
+func GetSupportedReduceFuncs() []string {
+	return []string{"sum", "mean", "min", "max", "count", "last"}
+}
+
 // Reduce turns the Series into a Number based on the given reduction function
 // if ReduceMapper is defined it applies it to the provided series and performs reduction of the resulting series.
 // Otherwise, the reduction operation is done against the original series.