monitoring information does not distinguish between internal and external transactions

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

`run the auto testcase and check the monitoring information on grafana，such as TIDB dashboard ---Transaction ---duration，We can see a small number of optimistic transactions above the second level, which are confirmed by slow logs and are internal analyze operations`

### 2. What did you expect to see? (Required)

`distinguish between internal and external transaction statistics in terms of monitoring information`

### 3. What did you see instead (Required)
`do not distinguish between internal and external transaction statistics in terms of monitoring information，may confuse users`

### 4. What is your TiDB version? (Required)

```
Release Version: v6.0.0-alpha
Edition: Community
Git Commit Hash: 74d74b0a7b6bee275d392199bb319859682627ff
Git Branch: heads/refs/tags/v6.0.0-alpha
UTC Build Time: 2022-03-04 06:12:33
GoVersion: go1.16.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
```