SSE: Change math expression to accept any value convertible to float (#34996)

* SSE: Change math expression to accept any scalar value

* Apply suggestions from code review

* Update test

* Remove TODO

pkg/expr/mathexp/exp_test.go

@@ -30,10 +30,27 @@ func makeNumber(name string, labels data.Labels, f *float64) Number {
 	return newNumber
 }
 
+func unixTimePointer(sec, nsec int64) *time.Time {
+	t := time.Unix(sec, nsec)
+	return &t
+}
+
 func float64Pointer(f float64) *float64 {
 	return &f
 }
 
+func strPointer(s string) *string {
+	return &s
+}
+
+func int64Pointer(i int64) *int64 {
+	return &i
+}
+
+func boolPointer(b bool) *bool {
+	return &b
+}
+
 var aSeries = Vars{
 	"A": Results{
 		[]Value{