Lost metric for AutoID allocation when auto_id_cache set to 1

[benmaoer]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

use tiup playground to start an cluster, create a db and a table:

mysql> create database sbtest;
Query OK, 0 rows affected (0.19 sec)

mysql> CREATE TABLE sbtest1 (
    ->   id int(11) NOT NULL AUTO_INCREMENT,
    ->   k int(11) NOT NULL DEFAULT '0',
    ->   c char(120) NOT NULL DEFAULT '',
    ->   pad char(60) NOT NULL DEFAULT '',
    ->   PRIMARY KEY (id) /*T![clustered_index] CLUSTERED */,
    ->   KEY k_1 (k)
    -> ) auto_id_cache = 1;
Query OK, 0 rows affected (0.15 sec)

use sysbench to do a test:

sysbench oltp_insert run --mysql-host=[127.0.0.1](http://127.0.0.1/) --mysql-port=4000 --mysql-user=root  --db-driver=mysql --mysql-db=sbtest --threads=50 --time=1200 --report-interval=10 --tables=1 --table-size=10000000 --mysql-password=

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

Could find AutoID allocation metrics on dashboard.

3. What did you see instead (Required)

The metrics of AutoID is empty.

4. What is your TiDB version? (Required)

mysql> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v6.5.0
Edition: Community
Git Commit Hash: 706c3fa3c526cdba5b3e9f066b1a568fb96c56e3
Git Branch: heads/refs/tags/v6.5.0
UTC Build Time: 2022-12-27 03:42:38
GoVersion: go1.19.3
Race Enabled: false
TiKV Min Version: 6.2.0-alpha
Check Table Before Drop: false
Store: tikv
1 row in set (0.00 sec)