Fix prometheus passthrough for existing value types (#3351)

plugins/inputs/prometheus/parser.go

@@ -86,7 +86,7 @@ func Parse(buf []byte, header http.Header) ([]telegraf.Metric, error) {
 				} else {
 					t = time.Now()
 				}
-				metric, err := metric.New(metricName, tags, fields, t)
+				metric, err := metric.New(metricName, tags, fields, t, valueType(mf.GetType()))
 				if err == nil {
 					metrics = append(metrics, metric)
 				}
@@ -97,6 +97,17 @@ func Parse(buf []byte, header http.Header) ([]telegraf.Metric, error) {
 	return metrics, err
 }
 
+func valueType(mt dto.MetricType) telegraf.ValueType {
+	switch mt {
+	case dto.MetricType_COUNTER:
+		return telegraf.Counter
+	case dto.MetricType_GAUGE:
+		return telegraf.Gauge
+	default:
+		return telegraf.Untyped
+	}
+}
+
 // Get Quantiles from summary metric
 func makeQuantiles(m *dto.Metric) map[string]interface{} {
 	fields := make(map[string]interface{})