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]

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• disksing
• nolouch

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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: ff080cc

[codecov]

Codecov Report

Merging #4117 (e33ff01) into release-5.1 (edfecb9) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-5.1    #4117      +/-   ##
===============================================
+ Coverage        75.23%   75.30%   +0.06%     
===============================================
  Files              245      245              
  Lines            24646    24657      +11     
===============================================
+ Hits             18543    18567      +24     
+ Misses            4489     4479      -10     
+ Partials          1614     1611       -3     

Flag	Coverage Δ
unittests	75.30% <100.00%> (+0.06%)	⬆️

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

Impacted Files	Coverage Δ
server/schedulers/balance_region.go	89.07% <100.00%> (+0.58%)	⬆️
server/schedulers/utils.go	94.14% <100.00%> (+0.14%)	⬆️
server/election/leadership.go	85.88% <0.00%> (-3.53%)	⬇️
server/schedulers/random_merge.go	60.65% <0.00%> (-3.28%)	⬇️
client/base_client.go	82.47% <0.00%> (-2.07%)	⬇️
server/tso/tso.go	71.91% <0.00%> (-1.13%)	⬇️
pkg/btree/btree.go	85.30% <0.00%> (-0.97%)	⬇️
server/server.go	71.58% <0.00%> (-0.61%)	⬇️
server/grpc_service.go	48.34% <0.00%> (-0.46%)	⬇️
server/tso/allocator_manager.go	80.30% <0.00%> (+0.16%)	⬆️
... and 8 more

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