statistics: remove h.mu.Lock when creating NewSessionStatsCollector (#44510)

[ti-chi-bot]

This is an automated cherry-pick of #44510

What problem does this PR solve?

Issue Number: close #44502

Problem Summary:

What is changed and how it works?

We don't need to h.mu.Lock when creating NewSessionStatsCollector or NewSessionIndexUsageCollector.

Check List

Tests

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

Use time.Sleep to simulate slow internal sql in dumpTableStatCountToKV. When dumpTableStatCountToKV is holding h.mu, MySQL client can still establish connection with TiDB server.

• 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]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chrysan, hawkingrei

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• OWNERS [chrysan,hawkingrei]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-08 06:22:30.399337495 +0000 UTC m=+158442.243972095: ☑️ agreed by chrysan.
• 2023-06-12 07:14:37.869062096 +0000 UTC m=+136627.323823396: ☑️ agreed by hawkingrei.

[hawkingrei]

/retest