fix(admin): refresh cluster metadata while adding storage node (#558)

### What this PR does

It fixes a transient error caused by fetching cluster metadata too lately while handling AddStorageNode. This PR lets the AddStorageNode RPC handler fetch the cluster metadata forcibly and wait for them to be updated. Although it isn't a critical error and can be resolved through a simple retry, it is resolved quickly by fetching the cluster metadata.

Error example:

```
--- FAIL: TestVarlogAppend (5.38s)
    controller.go:62:
                Error Trace:    /home/runner/work/varlog/varlog/tests/ee/controller/controller.go:62
                                                        /home/runner/work/varlog/varlog/tests/ee/cluster/local/cluster.go:238
                                                        /home/runner/work/varlog/varlog/tests/ee/cluster/local/cluster.go:63
                                                        /home/runner/work/varlog/varlog/tests/ee/ee_test.go:32
                Error:          Received unexpected error:
                                exit status 255
                Test:           TestVarlogAppend
                Messages:       /home/runner/work/varlog/varlog/bin/varlogctl storagenode add --storage-node-id 1 --storage-node-address 127.0.0.1:20001 --admin 127.0.0.1:9093: rpc error: code = Unavailable desc = add storage node: call again
```

internal/admin/admin.go

@@ -337,13 +337,33 @@ func (adm *Admin) addStorageNode(ctx context.Context, snid types.StorageNodeID,
 		return nil, status.Errorf(status.Code(err), "add storage node: %s", err.Error())
 	}
 
-	adm.snmgr.AddStorageNode(ctx, snmd.StorageNode.StorageNodeID, addr)
-	adm.statRepository.Report(ctx, snmd, now)
-	snm, ok := adm.statRepository.GetStorageNode(snid)
-	if !ok {
-		return nil, status.Error(codes.Unavailable, "add storage node: call again")
+	confirm := func() *admpb.StorageNodeMetadata {
+		adm.snmgr.AddStorageNode(ctx, snmd.StorageNode.StorageNodeID, addr)
+		_, _ = adm.mrmgr.ClusterMetadataView().ClusterMetadata(ctx) // Fetch new cluster metadata.
+		adm.statRepository.Report(ctx, snmd, now)
+		if snm, ok := adm.statRepository.GetStorageNode(snid); ok {
+			return snm
+		}
+		return nil
+	}
+
+	if snm := confirm(); snm != nil {
+		return snm, nil
+	}
+
+	const interval = 100 * time.Millisecond
+	ticker := time.NewTicker(interval)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ticker.C:
+			if snm := confirm(); snm != nil {
+				return snm, nil
+			}
+		case <-ctx.Done():
+			return nil, status.Error(codes.Unavailable, "add storage node: call again")
+		}
 	}
-	return snm, nil
 }
 
 func (adm *Admin) unregisterStorageNode(ctx context.Context, snid types.StorageNodeID) error {