metric tidb_server_maxprocs does not reflect performance.max-procs

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
Deploy TiDB v8.5+ on a host with 8 CPU.
Set performance.max-procs = 4 in tidb.toml and start TiDB.

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

metric tidb_server_maxprocs shows quota 400%

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

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

v8.5.5

| Release Version: v8.5.5
Edition: Community
Git Commit Hash: 1fa258b833ff113883beeba40bc130be7ce66610
Git Branch: HEAD
UTC Build Time: 2026-06-30 08:37:08
GoVersion: go1.25.5
Race Enabled: false
Check Table Before Drop: false
Store: tikv |

Start by reproducing the issue with TiDB v8.5+ on an 8-CPU host, setting performance.max-procs to 4 and checking tidb_server_maxprocs. Trace how the configuration value and metric are connected; done means the metric reports a 400% quota for that configuration.

Written by the indexing model from the issue text.