pkg: improve region statistics observe

[rleungx]

What problem does this PR solve?

Issue Number: ref #7897.

What is changed and how does it work?

$ benchstat old.txt new.txt
goos: linux
goarch: amd64
pkg: github.com/tikv/pd/pkg/statistics
cpu: Intel(R) Xeon(R) Gold 6240 CPU @ 2.60GHz
           │   old.txt   │              new.txt               │
           │   sec/op    │   sec/op     vs base               │
Observe-72   2.134µ ± 4%   2.012µ ± 7%  -5.69% (p=0.015 n=10)

           │  old.txt   │               new.txt               │
           │    B/op    │    B/op      vs base                │
Observe-72   485.0 ± 1%   325.5 ± 49%  -32.89% (p=0.003 n=10)

           │  old.txt   │              new.txt               │
           │ allocs/op  │ allocs/op   vs base                │
Observe-72   5.000 ± 0%   3.000 ± 0%  -40.00% (p=0.000 n=10)

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• nolouch

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.45%. Comparing base (59e29cc) to head (b0488a6).

❗ Current head b0488a6 differs from pull request most recent head c92932c

Please upload reports for the commit c92932c to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8181      +/-   ##
==========================================
+ Coverage   77.41%   77.45%   +0.03%     
==========================================
  Files         471      471              
  Lines       61363    61383      +20     
==========================================
+ Hits        47503    47543      +40     
+ Misses      10288    10280       -8     
+ Partials     3572     3560      -12     

Flag	Coverage Δ
unittests	77.45% <100.00%> (+0.03%)	⬆️

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[JmPotato]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: b0488a6

[ti-chi-bot]

@rleungx: Your PR was out of date, I have automatically updated it for you.

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