Use batch serialization format in exec output (#6446)

plugins/outputs/exec/exec.go

@@ -67,13 +67,11 @@ func (e *Exec) SampleConfig() string {
 // Write writes the metrics to the configured command.
 func (e *Exec) Write(metrics []telegraf.Metric) error {
 	var buffer bytes.Buffer
-	for _, metric := range metrics {
-		value, err := e.serializer.Serialize(metric)
+	serializedMetrics, err := e.serializer.SerializeBatch(metrics)
 	if err != nil {
 		return err
 	}
-		buffer.Write(value)
-	}
+	buffer.Write(serializedMetrics)
 
 	if buffer.Len() <= 0 {
 		return nil