design: add reading metrics to the design of multi-factor-balance

[djshow832]

What problem does this PR solve?

Issue Number: ref #465

Problem Summary:
Now TiProxy reads from backends when Prometheus is unavailable.

What is changed and how it works:
Update the design of multi-factor-balance

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

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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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  Hits            ?     5567           
  Misses          ?     2088           
  Partials        ?      397           

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-15 08:09:41.82080339 +0000 UTC m=+428266.524273056: ☑️ agreed by xhebox.