DDL: Skip collecting TiFlash status when TiFlash is down (#40872)

[ti-chi-bot]

This is an automated cherry-pick of #40872

What problem does this PR solve?

Issue Number: close #38484

Problem Summary:

What is changed and how it works?

Skip CollectTiFlashStatus when the state of TiFlash store is Down.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  Two TFlash nodes, keep a TiFlash down and then set tiflash replica. Expect that the available become 1.
• 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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[hehechen]

/cc @breezewish @windtalker

[windtalker]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: windtalker, XuHuaiyu

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-08-16 03:02:05.122444865 +0000 UTC m=+686489.671460837: ☑️ agreed by windtalker.
• 2023-08-16 03:04:21.526670596 +0000 UTC m=+686626.075686565: ☑️ agreed by XuHuaiyu.

[windtalker]

/run-unit-test

[windtalker]

/test unit-test

[tiprow]

@windtalker: No presubmit jobs available for pingcap/tidb@release-6.5

In response to this:

/test unit-test

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-6.5@50ce164). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

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