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:

plugins/outputs/librato/librato_test.go

@@ -139,12 +139,8 @@ func TestBuildGauge(t *testing.T) {
 		},
 		{
 			testutil.TestMetric("11234.5", "test7"),
-			&Gauge{
-				Name:        "value1.test7.value",
-				MeasureTime: time.Date(2009, time.November, 10, 23, 0, 0, 0, time.UTC).Unix(),
-				Value:       11234.5,
-			},
-			fmt.Errorf("unable to extract value from Fields, undeterminable type"),
+			nil,
+			nil,
 		},
 	}
 
@@ -158,6 +154,9 @@ func TestBuildGauge(t *testing.T) {
 			t.Errorf("%s: expected an error (%s) but none returned",
 				gt.ptIn.Name(), gt.err.Error())
 		}
+		if len(gauges) != 0 && gt.outGauge == nil {
+			t.Errorf("%s: unexpected gauge, %+v\n", gt.ptIn.Name(), gt.outGauge)
+		}
 		if len(gauges) == 0 {
 			continue
 		}