Fix latency metrics for async kafka producer

pkg/target/kafka.go

@@ -219,7 +219,8 @@ func (kt *KafkaTarget) Write(messages []*models.Message) (*models.TargetWriteRes
 	var errResult error
 
 	if kt.asyncProducer != nil {
-		// Not adding request latency metric to async producer for now, since it would complicate the implementation, and delay our debug.
+
+		requestStarted := time.Now().UTC()
 		for _, msg := range safeMessages {
 			kt.asyncProducer.Input() <- &sarama.ProducerMessage{
 				Topic:    kt.topicName,
@@ -243,6 +244,8 @@ func (kt *KafkaTarget) Write(messages []*models.Message) (*models.TargetWriteRes
 				if originalMessage.AckFunc != nil {
 					originalMessage.AckFunc()
 				}
+				originalMessage.TimeRequestStarted = requestStarted
+				originalMessage.TimeRequestFinished = time.Now().UTC()
 				sent = append(sent, originalMessage)
 			}
 		}