Improve backlogSize stats in the topic. (#6700)

### Motivation

When all subscriptions have no backlogs, but the backlog size of the topic stats is not 0. So this PR improves the backlog size calculation of the managed ledger.

### Modifications

If all entries are consumed, return the ledger size as the consumed size.

### Verifying this change

A new unit test added.

(cherry picked from commit d72e383b)

managed-ledger/src/main/java/org/apache/bookkeeper/mledger/impl/ManagedLedgerImpl.java

@@ -985,8 +985,12 @@ public class ManagedLedgerImpl implements ManagedLedger, CreateCallback {
         if (ledgerEntries <= 0) {
             return 0;
         }
-        long averageSize = ledgerSize / ledgerEntries;
-        return consumedEntries >= 0 ? (consumedEntries + 1) * averageSize : 0;
+        if (ledgerEntries == (consumedEntries + 1)) {
+            return ledgerSize;
+        } else {
+            long averageSize = ledgerSize / ledgerEntries;
+            return consumedEntries >= 0 ? (consumedEntries + 1) * averageSize : 0;
+        }
     }
 
     @Override

pulsar-broker/src/test/java/org/apache/pulsar/broker/admin/AdminApiTest.java

@@ -2249,4 +2249,31 @@ public class AdminApiTest extends MockedPulsarServiceBaseTest {
 
         Assert.assertTrue(admin.topics().getStats(topic).subscriptions.values().iterator().next().lastExpireTimestamp > 0L);
     }
+
+    @Test
+    public void testBacklogSizeShouldBeZeroWhenConsumerAckedAllMessages() throws Exception {
+        final String topic = "persistent://prop-xyz/ns1/testBacklogSizeShouldBeZeroWhenConsumerAckedAllMessages";
+        Consumer<byte[]> consumer = pulsarClient.newConsumer()
+                .topic(topic)
+                .subscriptionName("sub-1")
+                .subscribe();
+        Producer<byte[]> producer = pulsarClient.newProducer()
+                .topic(topic)
+                .create();
+
+        final int messages = 33;
+        for (int i = 0; i < messages; i++) {
+            producer.send(new byte[1024 * i * 5]);
+        }
+
+        for (int i = 0; i < messages; i++) {
+            consumer.acknowledgeCumulative(consumer.receive());
+        }
+
+        // Wait ack send
+        Thread.sleep(1000);
+
+        TopicStats topicStats = admin.topics().getStats(topic);
+        assertEquals(topicStats.backlogSize, 0);
+    }
 }