*: add new region score formula

[disksing]

Signed-off-by: disksing i@disksing.com

What problem does this PR solve?

Make region score grow smoother and less affected by available changes.

What is changed and how it works?

• introduce new formula (v2)
• add config to control formula version
• add test

Default formula for newly created clusters is "v2", all old clusters keeps "v1" unless set manually.

The test result shows that compared to v4.0.7, the new formula can help balance fast to converge and more stable even with a very high store limit configuration.

Check List

Tests

• Unit test

Related Issue

• Reduce latency jitter caused by balance pingcap/tidb#18012

Release note

• Introduce new region score formula

[codecov]

Codecov Report

Merging #3269 (186cc85) into master (16005cf) will decrease coverage by 0.04%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master    #3269      +/-   ##
==========================================
- Coverage   73.90%   73.85%   -0.05%     
==========================================
  Files         242      242              
  Lines       23004    23029      +25     
==========================================
+ Hits        17001    17009       +8     
- Misses       4426     4431       +5     
- Partials     1577     1589      +12     

Flag	Coverage Δ
unittests	73.85% <95.65%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/core/store.go	81.93% <90.90%> (+1.27%)	⬆️
pkg/mock/mockcluster/config.go	100.00% <100.00%> (ø)
server/api/config.go	65.87% <100.00%> (ø)
server/api/store.go	62.26% <100.00%> (ø)
server/config/config.go	74.41% <100.00%> (+0.10%)	⬆️
server/config/persist_options.go	91.33% <100.00%> (+0.03%)	⬆️
server/config/util.go	90.90% <100.00%> (ø)
server/schedule/filter/comparer.go	100.00% <100.00%> (ø)
server/schedulers/balance_region.go	85.84% <100.00%> (ø)
server/schedulers/utils.go	93.05% <100.00%> (+0.30%)	⬆️
... and 16 more

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[lhy1024]

LGTM, do we need to add some test result in this pr desc?

[nolouch]

lgtm

[disksing]

LGTM, do we need to add some test result in this pr desc?

ok, I'll add a short description.

[disksing]

/run-all-tests