You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: main.go
    path of file: ./repos/turn/examples/turn-client/udp
    
    the code of the file until where you have to start completion: // SPDX-FileCopyrightText: 2023 The Pion community <https://pion.ly>
// SPDX-License-Identifier: MIT

// Package main implements a TURN client using UDP
package main

import (
	"flag"
	"fmt"
	"log"
	"net"
	"strings"
	"time"

	"github.com/pion/logging"
	"github.com/pion/turn/v3"
)

func main() {
	host := flag.String("host", "", "TURN Server name.")
	port := flag.Int("port", 3478, "Listening port.")
	user := flag.String("user", "", "A pair of username and password (e.g. \"user=pass\")")
	realm := flag.String("realm", "pion.ly", "Realm (defaults to \"pion.ly\")")
	ping := flag.Bool("ping", false, "Run ping test")
	flag.Parse()

	if len(*host) == 0 {
		log.Fatalf("'host' is required")
	}

	if len(*user) == 0 {
		log.Fatalf("'user' is required")
	}

	cred := strings.SplitN(*user, "=", 2)

	// TURN client won't create a local listening socket by itself.
	conn, err := net.ListenPacket("udp4", "0.0.0.0:0")
	if err != nil {
		log.Panicf("Failed to listen: %s", err)
	}
	defer func() {
		if closeErr := conn.Close(); closeErr != nil {
			log.Panicf("Failed to close connection: %s", closeErr)
		}
	}()

	turnServerAddr := fmt.Sprintf("%s:%d", *hos
    
func doPingTest(client *turn.Client, relayConn net.PacketConn) error {
	// Send BindingRequest to learn our external IP
	mappedAddr, err := client.SendBindingRequest()
	if err != nil {
		return err
	}

	// Set up pinger socket (pingerConn)
	pingerConn, err := net.ListenPacket("udp4", "0.0.0.0:0")
	if err != nil {
		log.Panicf("Failed to listen: %s", err)
	}
	defer func() {
		if closeErr := pingerConn.Close(); closeErr != nil {
			log.Panicf("Failed to close connection: %s", closeErr)
		}
	}()

	// Punch a UDP hole for the relayConn by sending a data to the mappedAddr.
	// This will trigger a TURN client to generate a permission request to the
	// TURN server. After this, packets from the IP address will be accepted by
	// the TURN server.
	_, err = relayConn.WriteTo([]byte("Hello"), mappedAddr)
	if err != nil {
		return err
	}

	// Start read-loop on pingerConn
	go func() {
		buf := make([]byte, 1600)
		for {
			n, from, pingerErr := pingerConn.ReadFrom(buf)
			if pingerErr != nil {
				break
			}

			msg := string(buf[:n])
			if sentAt, pingerErr := time.Parse(time.RFC3339Nano, msg); pingerErr == nil {
				rtt := time.Since(sentAt)
				log.Printf("%d bytes from from %s time=%d ms\n", n, from.String(), int(rtt.Seconds()*1000))
			}
		}
	}()

	// Start read-loop on relayConn
	go func() {
		buf := make([]byte, 1600)
		for {
			n, from, readerErr := relayConn.ReadFrom(buf)
			if readerErr != nil {
				break
			}

			// Echo back
			if _, readerErr = relayConn.WriteTo(buf[:n], from); readerErr != nil {
				break
			}
		}
	}()

	time.Sleep(500 * time.Millisecond)

	// Send 10 packets from relayConn to the echo server
	for i := 0; i < 10; i++ {
		msg := time.Now().Format(time.RFC3339Nano)
		_, err = pingerConn.WriteTo([]byte(msg), relayConn.LocalAddr())
		if err != nil {
			return err
		}

		// For simplicity, this example does not wait for the pong (reply).
		// Instead, sleep 1 second.
		time.Sleep(time.Second)
	}

	return nil
}