feature(label): tolerate backslash in the label name (#1595)

[sre-bot]

cherry-pick #1595 to release-3.1

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Signed-off-by: Shafreeck Sea shafreeck@gmail.com

Closes #1588

What problem does this PR solve?

tolerate backslash in the label name.

What is changed and how it works?

relax the rules to check the validation of a label.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #2084 into release-3.1 will decrease coverage by 0.09%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           release-3.1    #2084     +/-   ##
==============================================
- Coverage        69.63%   69.53%   -0.1%     
==============================================
  Files              171      171             
  Lines            16848    16848             
==============================================
- Hits             11732    11716     -16     
- Misses            4144     4157     +13     
- Partials           972      975      +3

Impacted Files	Coverage Δ
server/util.go	72.61% <ø> (+1.91%)	⬆️
pkg/testutil/operator_check.go	83.33% <0%> (-16.67%)	⬇️
pkg/etcdutil/etcdutil.go	79.06% <0%> (-16.28%)	⬇️
server/schedulers/random_merge.go	62.5% <0%> (-5%)	⬇️
server/region_syncer/client.go	81.57% <0%> (-3.95%)	⬇️
server/schedule/operator.go	86.93% <0%> (-0.65%)	⬇️
server/cluster.go	81.31% <0%> (-0.51%)	⬇️
server/grpc_service.go	59.91% <0%> (-0.22%)	⬇️
client/client.go	74.74% <0%> (-0.21%)	⬇️
server/leader.go	83.26% <0%> (+1.22%)	⬆️
... and 3 more

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[lhy1024]

/merge

[sre-bot]

Your auto merge job has been accepted, waiting for 2083

[sre-bot]

/run-all-tests