Refactor: await the reporting of the replication status in the test c…

…ase 'add_learner_non_blocking'.

In this case, a replication failure status is supposed to be reported to
the matrix. But sometimes it times out before the failure report can be
delivered.

To address this issue, just extend the time out duration.

tests/tests/membership/t11_add_learner.rs

@@ -132,12 +132,27 @@ async fn add_learner_non_blocking() -> Result<()> {
         let raft = router.get_raft_handle(&0)?;
         raft.add_learner(1, (), false).await?;
 
-        sleep(Duration::from_millis(500)).await;
-
-        let metrics = router.get_raft_handle(&0)?.metrics().borrow().clone();
-        let repl = metrics.replication.as_ref().unwrap();
-        let n1_repl = repl.get(&1);
-        assert_eq!(Some(&None), n1_repl, "no replication state to the learner is reported");
+        let n = 6;
+        for i in 0..=n {
+            if i == n {
+                unreachable!("no replication status is reported to metrics!");
+            }
+
+            let metrics = router.get_raft_handle(&0)?.metrics().borrow().clone();
+            let repl = metrics.replication.as_ref().unwrap();
+
+            // The result is Some(&None) when there is no success replication is made,
+            // and is None if no replication attempt is made(no success or failure is reported to metrics).
+            let n1_repl = repl.get(&1);
+            if n1_repl.is_none() {
+                tracing::info!("--- no replication attempt is made, sleep and retry: {}-th attempt", i);
+
+                sleep(Duration::from_millis(500)).await;
+                continue;
+            }
+            assert_eq!(Some(&None), n1_repl, "no replication state to the learner is reported");
+            break;
+        }
     }
 
     Ok(())