*: support more system variables in statement summary (#15508)

[djshow832]

cherry-pick #15508 to release-4.0

---

What problem does this PR solve?

Problem Summary:max-stmt-count and max-sql-length were configurations in config file, but other configurations were system variables. It's not consistent and makes many users confused.

What is changed and how it works?

What's Changed:

1. Also support system variables tidb_stmt_summary_max_stmt_count and tidb_stmt_summary_max_sql_length.
2. Global default value is '', not the same with config value.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tidb-ansible:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

mysql> set global tidb_stmt_summary_max_stmt_count=50;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@tidb_stmt_summary_max_stmt_count;
+------------------------------------+
| @@tidb_stmt_summary_max_stmt_count |
+------------------------------------+
| 50                                 |
+------------------------------------+
1 row in set (0.00 sec)

mysql> set global tidb_stmt_summary_max_sql_length=50;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@tidb_stmt_summary_max_sql_length;
+------------------------------------+
| @@tidb_stmt_summary_max_sql_length |
+------------------------------------+
| 50                                 |
+------------------------------------+
1 row in set (0.00 sec)

Side effects

N/A

Release note

• Statement summary: support system variables tidb_stmt_summary_max_stmt_count and tidb_stmt_summary_max_sql_length.

[Deardrops]

LGTM

[lonng]

LGTM

[lonng]

/merge

[sre-bot]

Sorry @lonng, you don't have permission to trigger auto merge event on this branch.

[bb7133]

/merge

[sre-bot]

/run-all-tests

[zz-jason]

/merge

[sre-bot]

/run-all-tests