Storages: Add metrics in RateLimiter (#8636)

[ti-chi-bot]

This is an automated cherry-pick of #8636

What problem does this PR solve?

Issue Number: ref #8563

What is changed and how it works?

• Add metrics of pending count in rate limiter.
• Remove some useless code.

Check List

Tests

• Unit test

• Integration test

• Manual test (add detailed scripts or steps below)
  

• 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

None

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JinheLin, Lloyd-Pottiger

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Needs approval from an approver in each of these files:

• OWNERS [JinheLin,Lloyd-Pottiger]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-01-18 09:52:09.693842225 +0000 UTC m=+435971.258139930: ☑️ agreed by JinheLin.
• 2024-01-18 10:11:17.751442036 +0000 UTC m=+437119.315739739: ☑️ agreed by Lloyd-Pottiger.