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

[lhy1024]

What problem does this PR solve?

close #4497

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]

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[codecov]

Codecov Report

Merging #4106 (dfb5701) into master (3f53f1e) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

❗ Current head dfb5701 differs from pull request most recent head 3cd0569. Consider uploading reports for the commit 3cd0569 to get more accurate results

@@            Coverage Diff             @@
##           master    #4106      +/-   ##
==========================================
- Coverage   74.53%   74.46%   -0.08%     
==========================================
  Files         258      258              
  Lines       26323    26334      +11     
==========================================
- Hits        19621    19609      -12     
- Misses       4963     4981      +18     
- Partials     1739     1744       +5     

Flag	Coverage Δ
unittests	74.46% <100.00%> (-0.08%)	⬇️

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

Impacted Files	Coverage Δ
server/core/kind.go	61.90% <ø> (ø)
server/schedule/opt/opts.go	100.00% <ø> (ø)
pkg/mock/mockcluster/mockcluster.go	95.23% <100.00%> (+0.01%)	⬆️
server/cluster/cluster.go	82.58% <100.00%> (+0.37%)	⬆️
server/schedule/opt/healthy.go	100.00% <100.00%> (ø)
server/schedule/range_cluster.go	90.90% <100.00%> (+3.87%)	⬆️
server/schedulers/scatter_range.go	80.79% <100.00%> (ø)
server/schedulers/utils.go	94.75% <100.00%> (+0.07%)	⬆️
server/election/lease.go	84.12% <0.00%> (-4.77%)	⬇️
pkg/etcdutil/etcdutil.go	84.70% <0.00%> (-3.53%)	⬇️
... and 16 more

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[lhy1024]

/merge

[ti-chi-bot]

@lhy1024: 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: 03b045f

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4116.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4117.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4118.