MINOR: add the removed topic metrics to zk2kraft (apache#18658)

Reviewers: Christo Lolov <[email protected]>, Chia-Ping Tsai <[email protected]>

Author: m1a2st

docs/zk2kraft.html

@@ -176,5 +176,17 @@ <h5 class="anchor-heading">Removal metrics</h5>
                 Kafka remove all zookeeper dependencies, so the metrics is removed.
             </p>
         </li>
+        <li>
+            <p>
+                Remove the metrics which is monitoring requests waiting in the topic purgatory
+            </p>
+            <ul>
+                <li><code>kafka.server:type=DelayedOperationPurgatory,name=PurgatorySize,delayedOperation=topic</code></li>
+                <li><code>kafka.server:type=DelayedOperationPurgatory,name=NumDelayedOperations,delayedOperation=topic</code></li>
+            </ul>
+            <p>
+                In Kraft mode, Zookeeper is not used, so the metrics is removed.
+            </p>
+        </li>
     </ul>
 </div>