PD/DDL: Missing necessary metrics from etcd to identify the root cause of High 'Update Self Version Duration' when modifying column

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

Run continuous column modification DDLs on a TiDB cluster.

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

The "Update Self Version Duration" metric in the DDL subsystem should remain low and stable.

### 3. What did you see instead (Required)

The "Update Self Version Duration" metric consistently peaks at around 9 minutes.


There is no clear cause for this delay based on etcd metrics.



Note: etcd’s disk WAL fsync rate does increase during this period, but it shouldn't be enough to cause such a dramatic delay. And the continues MVCC and other gRPC durations remain normal and stable.

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

v7.5.6