some DXF metric is incorrect and might be un-registered, and cause `/metrics` API to return large mount of data

[D3Hunter]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

tidb_disttask_task_status might be incorrect, and tidb_disttask_start_time might not unregistered, cause the /metrics API keeps reporting it, if there are many tasks, and the TiDB keeps running, it might cause the API report large mount of data. this issue is already fixed by #59940
we record it for tracking

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

8.1