Use fieldsCopy in testutil.Accumulator AddFields (#4970)

testutil/accumulator.go

@@ -83,6 +83,10 @@ func (a *Accumulator) AddFields(
 		return
 	}
 
+	if len(fields) == 0 {
+		return
+	}
+
 	tagsCopy := map[string]string{}
 	for k, v := range tags {
 		tagsCopy[k] = v
@@ -93,10 +97,6 @@ func (a *Accumulator) AddFields(
 		fieldsCopy[k] = v
 	}
 
-	if len(fields) == 0 {
-		return
-	}
-
 	var t time.Time
 	if len(timestamp) > 0 {
 		t = timestamp[0]
@@ -114,7 +114,7 @@ func (a *Accumulator) AddFields(
 
 	p := &Metric{
 		Measurement: measurement,
-		Fields:      fields,
+		Fields:      fieldsCopy,
 		Tags:        tagsCopy,
 		Time:        t,
 	}