statistics: fix the bug that the isolation level is wrong when the store lacks label (#3467)

[ti-srebot]

cherry-pick #3467 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In pd repo:
git pr https://github.com/tikv/pd/pull/3474

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/pd.git pr/3474:release-4.0-3c81817831e9

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What problem does this PR solve?

close #3455

What is changed and how it works?

• statistics: fix the bug that the isolation level is wrong when the store lacks label

example

• pd location-labels: region,host
• tikv1 label: {host: 1}
• tikv2 label: {host: 2}
• tikv3 label: {host: 3}
• Before
  • 
• After
  • 

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

• fix the bug that the isolation level is wrong when the store lacks label

[ti-chi-bot]

[REVIEW NOTIFICATION]

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[ti-srebot]

@HunDunDM you're already a collaborator in bot's repo.

[HunDunDM]

/rebuild

[nolouch]

lgtm

[HunDunDM]

/merge

[ti-chi-bot]

@HunDunDM: 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: a0a2097

[codecov]

Codecov Report

Merging #3474 (a0a2097) into release-4.0 (3f81ab7) will increase coverage by 0.17%.
The diff coverage is 94.44%.

@@               Coverage Diff               @@
##           release-4.0    #3474      +/-   ##
===============================================
+ Coverage        74.69%   74.87%   +0.17%     
===============================================
  Files              209      209              
  Lines            19809    19811       +2     
===============================================
+ Hits             14797    14833      +36     
+ Misses            3674     3649      -25     
+ Partials          1338     1329       -9     

Flag	Coverage Δ
unittests	74.87% <94.44%> (+0.17%)	⬆️

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

Impacted Files	Coverage Δ
server/statistics/region_collection.go	93.85% <94.11%> (+4.57%)	⬆️
server/cluster/cluster.go	78.57% <100.00%> (ø)
pkg/tempurl/tempurl.go	60.00% <0.00%> (-10.00%)	⬇️
client/client.go	69.54% <0.00%> (+0.50%)	⬆️
server/member/leader.go	70.64% <0.00%> (+6.88%)	⬆️
pkg/etcdutil/etcdutil.go	85.29% <0.00%> (+7.35%)	⬆️
server/kv/etcd_kv.go	79.71% <0.00%> (+10.14%)	⬆️
pkg/errs/errs.go	100.00% <0.00%> (+25.00%)	⬆️

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