Reduce UniPS snapshot stat overhead (#7706)

[ti-chi-bot]

This is an automated cherry-pick of #7706

What problem does this PR solve?

Issue Number: close #7713

Problem Summary:

When there are many DeltaMergeStore in one tiflash instance, each DeltaMergeStore::getStoreStats will try to get the snapshot stats from PSV3/UniPS. But the PSV3/UniPS is a global instance that brings much CPU waste and locks contention. So it causes slow queries.

What is changed and how it works?

• Only get snapshot stats under PageStorageRunMode::ONLY_V2, because PSV2 have different PS instances for different table
• Reduce the lock scope of read_write_mutex in DeltaMergeStore::getStoreStats
• Add metrics about disagg RPC requests
• Refine some loggings

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

None

[JaySon-Huang]

/run-all-tests

[JaySon-Huang]

/rebuild

[JaySon-Huang]

/run-integration-test

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JaySon-Huang, lidezhu

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• OWNERS [JaySon-Huang,lidezhu]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-30 09:01:53.143241406 +0000 UTC m=+76196.517819242: ☑️ agreed by JaySon-Huang.
• 2023-06-30 09:09:20.657372057 +0000 UTC m=+76644.031949908: ☑️ agreed by lidezhu.