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: d4402f5

[codecov-commenter]

Codecov Report

Merging #3032 (d4402f5) into release-4.0 (753073a) will increase coverage by 3.3297%.
The diff coverage is 66.4204%.

@@                 Coverage Diff                 @@
##           release-4.0      #3032        +/-   ##
===================================================
+ Coverage      53.7661%   57.0959%   +3.3297%     
===================================================
  Files              153        164        +11     
  Lines            15958      19476      +3518     
===================================================
+ Hits              8580      11120      +2540     
- Misses            6475       7298       +823     
- Partials           903       1058       +155