Raise error if no threshold is provided for a numeric value

src/metrics-alerting/process/process.go

 	labels map[string]string,
 	data script_data.Data,
 ) error {
+	if script.Threshold == 0 {
+		return fmt.Errorf("no threshold provided for number value")
+	}
+
 	if value < script.Threshold {
 		// Nothing to alert about
 		return nil