[LOG4J2-2727] Add KafkaAppender.Builder#setKey
diff --git a/log4j-kafka/src/main/java/org/apache/logging/log4j/kafka/appender/KafkaAppender.java b/log4j-kafka/src/main/java/org/apache/logging/log4j/kafka/appender/KafkaAppender.java
index 41e3fc8..8f7de26 100644
--- a/log4j-kafka/src/main/java/org/apache/logging/log4j/kafka/appender/KafkaAppender.java
+++ b/log4j-kafka/src/main/java/org/apache/logging/log4j/kafka/appender/KafkaAppender.java
@@ -85,6 +85,11 @@
             return asBuilder();
         }
 
+        public B setKey(final String key) {
+            this.key = key;
+            return asBuilder();
+        }
+
         public B setSyncSend(final boolean syncSend) {
             this.syncSend = syncSend;
             return asBuilder();