metric: add alert for TiFlash down and remove alert about TMT.

[LittleFall]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

What is changed and how it works?

Proposal: xxx

What's Changed: as the title says.

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch:

Check List

Tests

• Manual test (add detailed scripts or steps below)

1. all nodes up
   

2. nodes down less than 1 minute.
   

3. nodes down more than 1 minute.
   

Side effects

Release note

Add alert for TiFlash down.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• zanmato1984

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[zanmato1984]

LGTM

[LittleFall]

/run-all-tests

[LittleFall]

/merge

[ti-chi-bot]

@LittleFall: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: b23ac49