Import stage is missing from the monitoring panel

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
For the same data, `import into from select` has no import stage speed in grafana.


### 2. What did you expect to see? (Required)
Import stage has no speed curve.
### 3. What did you see instead (Required)
Show import stage speed normally.
### 4. What is your TiDB version? (Required)
v8.1.2
```
mysql> select tidb_version()\G;
*************************** 1. row ***************************
tidb_version(): Release Version: v8.1.2
Edition: Community
Git Commit Hash: 80db67052c0b1d89ab5ea94d6efa576a2191432d
Git Branch: HEAD
UTC Build Time: 2024-12-24 06:39:24
GoVersion: go1.21.10
Race Enabled: false
Check Table Before Drop: false
Store: tikv
```