add verifyValue func for datadog and librato, bail if no good

closes #906

plugins/outputs/librato/librato.go

@@ -165,6 +165,9 @@ func (l *Librato) buildGauges(m telegraf.Metric) ([]*Gauge, error) {
 			Name:        l.buildGaugeName(m, fieldName),
 			MeasureTime: m.Time().Unix(),
 		}
+		if !gauge.verifyValue(value) {
+			continue
+		}
 		if err := gauge.setValue(value); err != nil {
 			return gauges, fmt.Errorf("unable to extract value from Fields, %s\n",
 				err.Error())
@@ -186,6 +189,14 @@ func (l *Librato) buildGauges(m telegraf.Metric) ([]*Gauge, error) {
 	return gauges, nil
 }
 
+func (g *Gauge) verifyValue(v interface{}) bool {
+	switch v.(type) {
+	case string:
+		return false
+	}
+	return true
+}
+
 func (g *Gauge) setValue(v interface{}) error {
 	switch d := v.(type) {
 	case int: