kv/client: add cached regions metric in region router (#2994)

[ti-chi-bot]

This is an automated cherry-pick of #2994

What problem does this PR solve?

Close #2985

What is changed and how it works?

• Add metric to record cached region count in region router.
• Add Grafana metric to estimate incremental remaining time, sample:

Check

List

Tests

• Unit test
• Integration test

Release note

Add metrics to observe incremental scan remaining time

[ti-chi-bot]

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[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: 4e4db07

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-5.1@38b4871). Click here to learn what that means.
The diff coverage is n/a.

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