KAFKA-19503: Deprecate MX4j support (#20208)

This feature adds maintenance burden and potential security concerns
while providing no apparent value to the Kafka community. See
[KIP-1193](https://cwiki.apache.org/confluence/x/dAxJFg) for more
details.

Reviewers: TengYao Chi <frankvicky@apache.org>, Ken Huang
 <s7133700@gmail.com>

---------

Signed-off-by: Federico Valeri <fedevaleri@gmail.com>

core/src/main/scala/kafka/utils/Mx4jLoader.scala

@@ -30,12 +30,14 @@ import javax.management.ObjectName
  *
  * This is a Scala port of org.apache.cassandra.utils.Mx4jTool written by Ran Tavory for CASSANDRA-1068
  * */
+@deprecated
 object Mx4jLoader extends Logging {
 
   def maybeLoad(): Boolean = {
     val props = new VerifiableProperties(System.getProperties)
     if (!props.getBoolean("kafka_mx4jenable", default = false))
       return false
+    warn("MX4j is deprecated and will be removed in Kafka 5.0")
     val address = props.getString("mx4jaddress", "0.0.0.0")
     val port = props.getInt("mx4jport", 8082)
     try {

docs/upgrade.html

@@ -34,9 +34,11 @@
     <li>The <code>KafkaPrincipalBuilder</code> now extends <code>KafkaPrincipalSerde</code>. Force developer to implement <code>KafkaPrincipalSerde</code> interface for custom <code>KafkaPrincipalBuilder</code>.
         For further details, please refer to <a href="https://cwiki.apache.org/confluence/x/1gq9F">KIP-1157</a>.
     </li>
+    <li>
+        The support for MX4J library, enabled through <code>kafka_mx4jenable</code> system property, was deprecated and will be removed in Kafka 5.0.
+    </li>
 </ul>
 
-
 <h4><a id="upgrade_4_1_0" href="#upgrade_4_1_0">Upgrading to 4.1.0</a></h4>
 
 <h5><a id="upgrade_4_1_0_from" href="#upgrade_4_1_0_from">Upgrading Servers to 4.1.0 from any version 3.3.x through 4.0.x</a></h5>