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

[HunDunDM]

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

[codecov]

Codecov Report

Merging #3467 (5d52a62) into master (2cf8161) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3467      +/-   ##
==========================================
- Coverage   75.11%   75.06%   -0.05%     
==========================================
  Files         244      244              
  Lines       23583    23585       +2     
==========================================
- Hits        17715    17705      -10     
- Misses       4292     4305      +13     
+ Partials     1576     1575       -1     

Flag	Coverage Δ
unittests	75.06% <100.00%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
server/cluster/cluster.go	83.17% <100.00%> (ø)
server/statistics/region_collection.go	97.08% <100.00%> (+1.52%)	⬆️
pkg/errs/errs.go	75.00% <0.00%> (-25.00%)	⬇️
pkg/dashboard/adapter/manager.go	82.97% <0.00%> (-7.45%)	⬇️
server/tso/allocator_manager.go	76.41% <0.00%> (-2.50%)	⬇️
server/schedule/operator/step.go	67.44% <0.00%> (-1.67%)	⬇️
pkg/btree/btree.go	85.30% <0.00%> (-0.97%)	⬇️
client/base_client.go	83.23% <0.00%> (-0.58%)	⬇️
server/grpc_service.go	57.59% <0.00%> (-0.51%)	⬇️
client/client.go	67.19% <0.00%> (+0.17%)	⬆️
... and 5 more

---

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[nolouch]

lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JmPotato
• nolouch

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[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

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[ti-chi-bot]

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

Commit hash: 5d52a62

[ti-srebot]

cherry pick to release-3.0 in PR #3473

[ti-srebot]

cherry pick to release-4.0 in PR #3474