RFC: Multi-Factor-Based Balance

[djshow832]

What problem does this PR solve?

Issue Number: ref #465

Problem Summary:

It may be inefficient to merely balance based on health and connection counts:

• The workload of each connection may be different: some may be busy while others may be idle. We have seen cases when connection counts are balanced while CPU usage is not.
• When some big queries make a TiDB OOM, all the connections on the TiDB are disconnected. Actually, connections without big queries have a chance to be migrated away before the TiDB fails.
• In multi-AZ deployment, the cross-AZ traffic between TiProxy and TiDB may lead to significant fees. It's more cost-effective for TiProxy to route to the local TiDB preferentially.

What is changed and how it works:

Proposes an RFC to balance TiDB instances based on more factors to maximize resource usage and reduce costs.

Some details are TBD, but the rough design is done.

Check List

Tests

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

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

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

None

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-06-11 02:13:27.033300498 +0000 UTC m=+430761.086612426: ☑️ agreed by xhebox.

[xhebox]

/hold

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: joey-yez, xhebox

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[djshow832]

/unhold