ddl: optimize temp index worker in highly conflicting case (#61445)

[ti-chi-bot]

This is an automated cherry-pick of #61445

What problem does this PR solve?

Issue Number: close #61433

Problem Summary:

When there are lots of conflicts between DDL internal txn and DML txn, temp index worker can hardly make any progress.

What changed and how does it work?

• Remove the LockKeys in DDL internal txn from merge index worker.
  • if both DDL txn and DML txn modify the same index key, there will be a conflict and DDL txn will retry. (after #62387)
  • if DDL txn modifies the index key, DML txn only modifies the row key, there will be no conflict anymore (this is the key improvement).
• Delete temp index records after applying them to the original index to eliminate reentrant cost.
• Reduce the batch count of DDL txn when suffering frequent conflicts.
• Add metrics to monitor the progress of merging temp index:
  • tidb_ddl_temp_index_write: the counter of writing to temp index (single write).
  • tidb_ddl_temp_index_double_write: the counter of writing to temp index (double write).
  • tidb_ddl_temp_index_scan: the counter of scanned temp index value records.
  • tidb_ddl_temp_index_merge: the counter of merged temp index value records.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

@tangenta This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/build	5e8f869	link	true	/test build
idc-jenkins-ci-tidb/unit-test	5e8f869	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev_2	5e8f869	link	true	/test check-dev2
idc-jenkins-ci-tidb/mysql-test	5e8f869	link	true	/test mysql-test
idc-jenkins-ci-tidb/check_dev	5e8f869	link	true	/test check-dev

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