scheduler: allow empty region to be scheduled and use a sperate tolerance config in scatter range scheduler (#4106)

[ti-chi-bot]

This is an automated cherry-pick of #4106

What problem does this PR solve?

1. we expect the tolerance ratio in range cluster should be 2, but it doesn't take effect.
   

   pd/server/schedulers/scatter_range.go

   Line 220 in 3f53f1e

   c.SetTolerantSizeRatio(2)

   
   

   pd/server/schedulers/utils.go

   Line 154 in 3f53f1e

   tolerantSizeRatio := cluster.GetOpts().GetTolerantSizeRatio()

2. we expect it can schedule empty region, but it can not and test can not detect
   

   pd/server/core/region.go

   Line 96 in 3f53f1e

   EmptyRegionApproximateSize = 1

   
   

   pd/server/schedulers/balance_test.go

   Lines 1103 to 1111 in 3f53f1e

   	// empty case
   	regions[49].EndKey = []byte("")
   	for _, meta := range regions {
   	leader := rand.Intn(4) % 3
   	regionInfo := core.NewRegionInfo(
   	meta,
   	meta.Peers[leader],
   	core.SetApproximateKeys(96),
   	core.SetApproximateSize(96),

What is changed and how it works?

allow empty region to be scheduled and use a sperate tolerance config in scatter range scheduler

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

Add scatter range scheduler

Before this pr

After this pr

Related changes

• Need to cherry-pick to the release branch

Release note

allow empty region to be scheduled and use a sperate tolerance config in scatter range scheduler

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• disksing
• nolouch

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[codecov]

Codecov Report

Merging #4118 (bcce372) into release-5.2 (24d1383) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-5.2    #4118      +/-   ##
===============================================
+ Coverage        74.65%   74.72%   +0.07%     
===============================================
  Files              249      249              
  Lines            25539    25550      +11     
===============================================
+ Hits             19065    19092      +27     
+ Misses            4788     4772      -16     
  Partials          1686     1686              

Flag	Coverage Δ
unittests	74.72% <100.00%> (+0.07%)	⬆️

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

Impacted Files	Coverage Δ
server/schedulers/balance_region.go	88.69% <100.00%> (+0.62%)	⬆️
server/schedulers/utils.go	94.46% <100.00%> (+0.08%)	⬆️
server/schedulers/shuffle_hot_region.go	55.55% <0.00%> (-10.11%)	⬇️
server/tso/local_allocator.go	71.62% <0.00%> (-6.76%)	⬇️
pkg/etcdutil/etcdutil.go	84.70% <0.00%> (-3.53%)	⬇️
pkg/dashboard/adapter/manager.go	79.78% <0.00%> (-3.20%)	⬇️
server/schedule/operator/step.go	67.44% <0.00%> (-1.67%)	⬇️
client/base_client.go	82.81% <0.00%> (-1.57%)	⬇️
server/tso/tso.go	60.22% <0.00%> (-1.11%)	⬇️
server/schedule/operator_controller.go	84.75% <0.00%> (-0.80%)	⬇️
... and 10 more

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[nolouch]

/merge

[ti-chi-bot]

@nolouch: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

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

Commit hash: e592319