Creating a producer interceptor for Kafka in Golang

package interceptor

import (
	"log"

	"github.com/IBM/sarama"
)

type ErrorTopic struct {
	Topic        string
	BlockedUsers map[string]struct{}
}

func (e ErrorTopic) OnSend(msg *sarama.ProducerMessage) {
	if msg.Topic != e.Topic {
		return
	}

	body, err := msg.Value.Encode()
	if err != nil {
		log.Println(err)

		return
	}

	if _, ok := e.BlockedUsers[string(body)]; !ok {
		return
	}

	msg.Headers = append(msg.Headers, sarama.RecordHeader{
		Key:   []byte("blocked"),
		Value: []byte("true"),
	})
}

package main

import (
	"log"
	"net/http"
	"time"

	"client/interceptor"

	"github.com/IBM/sarama"
)

func main() {
	// Create Kafka config.
	cfg := sarama.NewConfig()
	cfg.ClientID = "inanzzz"
	cfg.Version = sarama.V3_3_2_0
	cfg.Metadata.AllowAutoTopicCreation = false
	cfg.Producer.Return.Successes = true
	cfg.Producer.Compression = sarama.CompressionZSTD
	cfg.Producer.Interceptors = []sarama.ProducerInterceptor{
		interceptor.ErrorTopic{
			Topic:        "errors-topic",
			BlockedUsers: map[string]struct{}{"1": {}},
		},
	}

	// Create Kafka producer.
	pro, err := sarama.NewSyncProducer([]string{":9093"}, cfg)
	if err != nil {
		log.Fatalln(err)
	}
	defer pro.Close()

	// Create HTTP router.
	rtr := http.DefaultServeMux
	rtr.HandleFunc("GET /{topic}/{id}", (api{pro}).Handle)

	// Start HTTP server.
	http.ListenAndServe(":3001", rtr)
}

type api struct {
	sarama.SyncProducer
}

func (a api) Handle(w http.ResponseWriter, r *http.Request) {
	a.SendMessage(&sarama.ProducerMessage{
		Topic:     r.PathValue("topic"),
		Value:     sarama.StringEncoder(r.PathValue("id")),
		Timestamp: time.Now(),
	})
}