raftstore-v2: optimize the slowness detection on the Io-Hang scenario.

[LykxSassinator]

What is changed and how it works?

Issue Number: Ref #15070

What's Changed:

This pr includes:
* Polish the annotations in `raftstore-v2/src/worker/pd/slowness.rs`.
* Optimize the detection and the interval of reporting statistics on the io hang case.

Related changes

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

Check List

Tests

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

Side effects

• Performance regression
  • Consumes more CPU
  • Consumes more MEM
• Breaking backward compatibility

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• innerr
• overvenus

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[LykxSassinator]

/cc @tonyxuqqi @innerr

[LykxSassinator]

/cc @overvenus

PTAL, thx

[innerr]

LGTM

[overvenus]

/merge

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[ti-chi-bot]

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

Commit hash: b258b22