Some metrics for DDL Backfill need to be optimized in million tables scenario

[lilinghai]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

In the million tables and many ddl(add index/change column type) scenrio, the tidb_ddl_add_index_total and tidb_ddl_backfill_percentage_progress metrics will record all the table ddl backfill data.
The amount of data is too large, and The Grafana needs to consume a significant amount of resources to display this monitoring, which may even cause the browser to freeze.

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

Release Version: v8.5.0-alpha-299-g56c07d07a7
Edition: Community
Git Commit Hash: 56c07d0
Git Branch: HEAD
UTC Build Time: 2024-12-08 08:54:55
GoVersion: go1.23.4
Race Enabled: false
Check Table Before Drop: false
Store: tikv