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/testing.go

@@ -0,0 +1,18 @@
+package mathexp
+
+import (
+	"math/rand"
+
+	"github.com/grafana/grafana/pkg/util"
+)
+
+func GenerateNumber(value *float64) Number {
+	size := rand.Intn(5)
+	labels := make(map[string]string, size)
+	for i := 0; i < size; i++ {
+		labels[util.GenerateShortUID()] = util.GenerateShortUID()
+	}
+	result := NewNumber(util.GenerateShortUID(), labels)
+	result.SetValue(value)
+	return result
+}