statistics: merge global stats even if some partition stats are missing (#41176)

[ti-chi-bot]

This is an automated cherry-pick of #41176

What problem does this PR solve?

Issue Number: ref #38999

Problem Summary:

What is changed and how it works?

Introduce a system variable tidb_skip_missing_partition_stats, whose default value is on. If tidb_skip_missing_partition_stats turns on, merging global stats would skip partitions whose stats are missing rather than fail.

Check List

Tests

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

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]

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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/check_dev_2	ca2b28e	link	true	/test check-dev2
idc-jenkins-ci-tidb/build	ca2b28e	link	true	/test build
idc-jenkins-ci-tidb/check_dev	ca2b28e	link	true	/test check-dev
idc-jenkins-ci-tidb/unit-test	ca2b28e	link	true	/test unit-test
idc-jenkins-ci-tidb/mysql-test	ca2b28e	link	true	/test mysql-test

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