Fix "N instances should be drained but only M instances was drained"

This commit fixes the following error:

```
Error: failed to replace instances:
    github.com/abicky/ecsmec/cmd.newRuntimeError
        github.com/abicky/ecsmec/cmd/root.go:38
  - failed to terminate instances:
    github.com/abicky/ecsmec/internal/capacity.(*AutoScalingGroup).ReplaceInstances
        github.com/abicky/ecsmec/internal/capacity/autoscalinggroup.go:64
  - failed to drain instances:
    github.com/abicky/ecsmec/internal/capacity.(*AutoScalingGroup).terminateInstances
        github.com/abicky/ecsmec/internal/capacity/autoscalinggroup.go:212
  - 9 instances should be drained but only 8 instances was drained:
    github.com/abicky/ecsmec/internal/capacity.(*drainer).Drain
        github.com/abicky/ecsmec/internal/capacity/drainer.go:75
```

internal/capacity/drainer.go

@@ -55,7 +55,7 @@ func (d *drainer) Drain(ctx context.Context, instanceIDs []string) error {
 		processedCount += len(instances)
 
 		arns := make([]*string, len(instances))
-		fmt.Printf("Drain the following container instances in the cluster \"%s\":\n", d.cluster)
+		fmt.Printf("Drain the following %d container instances in the cluster \"%s\":\n", len(instances), d.cluster)
 		for i, instance := range instances {
 			arns[i] = instance.ContainerInstanceArn
 			fmt.Printf("\t%s (%s)\n", getContainerInstanceID(*instance.ContainerInstanceArn), *instance.Ec2InstanceId)
@@ -72,7 +72,7 @@ func (d *drainer) Drain(ctx context.Context, instanceIDs []string) error {
 		return xerrors.Errorf("no target instances exist in the cluster \"%s\"", d.cluster)
 	}
 	if processedCount != len(instanceIDs) {
-		return xerrors.Errorf("%d instances should be drained but only %d instances was drained", len(instanceIDs), processedCount)
+		return xerrors.Errorf("%d instances should be drained but only %d instances were drained", len(instanceIDs), processedCount)
 	}
 
 	return nil
@@ -226,11 +226,14 @@ func (d *drainer) drainContainerInstances(ctx context.Context, arns []*string, w
 
 func (d *drainer) processContainerInstances(ctx context.Context, instanceIDs []string, callback func([]ecstypes.ContainerInstance) error) error {
 	params := &ecs.ListContainerInstancesInput{
-		Cluster:    aws.String(d.cluster),
-		Filter:     aws.String(fmt.Sprintf("ec2InstanceId in [%s]", strings.Join(instanceIDs, ","))),
-		MaxResults: aws.Int32(d.batchSize),
+		Cluster: aws.String(d.cluster),
+		Filter:  aws.String(fmt.Sprintf("ec2InstanceId in [%s]", strings.Join(instanceIDs, ","))),
+	}
+	if err := d.waitUntilContainerInstancesRegistered(ctx, len(instanceIDs), params); err != nil {
+		return xerrors.Errorf("failed to wait until container instances are registered: %w", err)
 	}
 
+	params.MaxResults = aws.Int32(d.batchSize)
 	paginator := ecs.NewListContainerInstancesPaginator(d.ecsSvc, params)
 	for paginator.HasMorePages() {
 		page, err := paginator.NextPage(ctx)
@@ -250,13 +253,44 @@ func (d *drainer) processContainerInstances(ctx context.Context, instanceIDs []s
 		}
 
 		if err := callback(resp.ContainerInstances); err != nil {
-			return xerrors.Errorf("failed to list container instances: %w", err)
+			return xerrors.Errorf("failed to execute the callback: %w", err)
 		}
 	}
 
 	return nil
 }
 
+func (d *drainer) waitUntilContainerInstancesRegistered(ctx context.Context, count int, params *ecs.ListContainerInstancesInput) error {
+	ticker := time.NewTicker(10 * time.Second)
+	defer ticker.Stop()
+
+	timeout := 5 * time.Minute
+	timer := time.NewTimer(timeout)
+	defer timer.Stop()
+
+	for {
+		instanceCount := 0
+		paginator := ecs.NewListContainerInstancesPaginator(d.ecsSvc, params)
+		for paginator.HasMorePages() {
+			page, err := paginator.NextPage(ctx)
+			if err != nil {
+				return xerrors.Errorf("failed to list container instances: %w", err)
+			}
+			instanceCount += len(page.ContainerInstanceArns)
+		}
+		if instanceCount == count {
+			return nil
+		}
+
+		select {
+		case <-ticker.C:
+			continue
+		case <-timer.C:
+			return xerrors.Errorf("%d container instances expect to be registered but only %d instances were registered within %v", count, instanceCount, timeout)
+		}
+	}
+}
+
 func getContainerInstanceID(arn string) string {
 	return arn[strings.LastIndex(arn, "/")+1:]
 }