Add comment for legacy

src/DDTrace/Integrations/Kafka/KafkaIntegration.php

@@ -69,13 +69,22 @@ public static function addProducerSpanMetadata($span, $conf, $args)
 
     private function injectHeadersIntoArgs(array $args, array $headers): array
     {
+        // public RdKafka\ProducerTopic::producev (
+        //      integer $partition ,
+        //      integer $msgflags ,
+        //      string $payload [,
+        //      string $key = NULL [,
+        //      array $headers = NULL [,
+        //      integer $timestamp_ms = NULL [,
+        //      string $opaque = NULL
+        // ]]]] ) : void
         $argsCount = count($args);
         if ($argsCount >= 5) {
             $args[4] = array_merge($args[4] ?? [], $headers);
         } elseif ($argsCount === 4) {
             $args[] = $headers;
-        } else {
-            $args[] = null;
+        } else { // $argsCount === 3
+            $args[] = null;  // $key
             $args[] = $headers;
         }
         return $args;

tests/Integrations/Kafka/KafkaTest.php

@@ -152,7 +152,7 @@ public function testSpanLinksHighLevel()
         self::putEnv('DD_TRACE_DEBUG_PRNG_SEED=42');
 
         list($producerTraces, $output) = $this->inCli(
-            __DIR__ . '/scripts/producer.php',
+            __DIR__ . '/scripts/producerv.php',
             [
                 'DD_TRACE_AUTO_FLUSH_ENABLED' => 'true',
                 'DD_TRACE_GENERATE_ROOT_SPAN' => 'false',

tests/Integrations/Kafka/scripts/producerv.php

@@ -0,0 +1,77 @@
+<?php
+
+declare(strict_types=1);
+
+use RdKafka\Conf;
+use RdKafka\Message;
+use RdKafka\Producer;
+use RdKafka\TopicConf;
+
+$conf = new Conf();
+$conf->set('bootstrap.servers', 'kafka_integration:9092');
+$conf->set('socket.timeout.ms', (string) 50);
+$conf->set('queue.buffering.max.messages', (string) 1000);
+$conf->set('max.in.flight.requests.per.connection', (string) 1);
+$conf->setDrMsgCb(
+    function (Producer $producer, Message $message): void {
+        if ($message->err !== RD_KAFKA_RESP_ERR_NO_ERROR) {
+            //var_dump($message->errstr());
+        }
+        //var_dump($message);
+    }
+);
+//$conf->set('log_level', (string) LOG_DEBUG);
+//$conf->set('debug', 'all');
+$conf->setLogCb(
+    function (Producer $producer, int $level, string $facility, string $message): void {
+        echo sprintf('log: %d %s %s', $level, $facility, $message) . PHP_EOL;
+    }
+);
+$conf->set('statistics.interval.ms', (string) 1000);
+$conf->setStatsCb(
+    function (Producer $producer, string $json, int $json_len, $opaque = null): void {
+        echo "stats: ${json}" . PHP_EOL;
+    }
+);
+$conf->setDrMsgCb(function (Producer $kafka, Message $message) {
+    if (RD_KAFKA_RESP_ERR_NO_ERROR !== $message->err) {
+        $errorStr = rd_kafka_err2str($message->err);
+
+        echo sprintf('Message FAILED (%s, %s) to send with payload => %s', $message->err, $errorStr, $message->payload) . PHP_EOL;
+    } else {
+        // message successfully delivered
+        echo sprintf('Message sent SUCCESSFULLY with payload => %s', $message->payload) . PHP_EOL;
+    }
+});
+//var_dump($conf->dump());
+
+$topicConf = new TopicConf();
+$topicConf->set('message.timeout.ms', (string) 30000);
+$topicConf->set('request.required.acks', (string) -1);
+$topicConf->set('request.timeout.ms', (string) 5000);
+//var_dump($topicConf->dump());
+
+$producer = new Producer($conf);
+
+$topicName = $argv[1] ?? 'test';
+$topic = $producer->newTopic($topicName, $topicConf);
+//var_dump($topic);
+
+$metadata = $producer->getMetadata(false, $topic, 1000);
+//var_dump($metadata->getOrigBrokerName());
+//var_dump($metadata->getOrigBrokerId());
+//var_dump($metadata->getBrokers());
+//var_dump($metadata->getTopics());
+
+
+for ($i = 0; $i < 1; $i++) {
+    $key = $i % 10;
+    $payload = sprintf('payload-%d-%s', $i, $key);
+    echo sprintf('produce msg: %s', $payload) . PHP_EOL;
+    $topic->producev(RD_KAFKA_PARTITION_UA, 0, $payload, (string) $key);
+    // triggers log output
+    $events = $producer->poll(1);
+    echo sprintf('polling triggered %d events', $events) . PHP_EOL;
+}
+
+$producer->flush(5000);