Make getFirstMetricValue to return the first value

This function was previously returning the last value instead of the
first value. This fix changes it and returns the value as soon as
reading it.

input/system/azure/system.go

@@ -286,6 +286,7 @@ func getFirstMetricValue(metric *azquery.Metric) (metricValue *azquery.MetricVal
 	for _, timeSeriesElement := range metric.TimeSeries {
 		for _, mValue := range timeSeriesElement.Data {
 			metricValue = mValue
+			return
 		}
 	}
 	return