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

[sre-bot]

cherry-pick #1595 to release-3.0

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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 #2083 into release-3.0 will increase coverage by 0.03%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           release-3.0    #2083      +/-   ##
===============================================
+ Coverage        69.57%   69.61%   +0.03%     
===============================================
  Files              170      170              
  Lines            16802    16802              
===============================================
+ Hits             11690    11696       +6     
+ Misses            4141     4139       -2     
+ Partials           971      967       -4

Impacted Files	Coverage Δ
server/util.go	75.15% <ø> (+2.54%)	⬆️
server/schedulers/random_merge.go	62.5% <0%> (-5%)	⬇️
server/leader.go	78.77% <0%> (-0.82%)	⬇️
server/cluster.go	81.72% <0%> (-0.51%)	⬇️
server/handler.go	51.37% <0%> (-0.51%)	⬇️
server/grpc_service.go	59.91% <0%> (-0.22%)	⬇️
client/client.go	74.73% <0%> (ø)	⬆️
server/region_syncer/client.go	85.52% <0%> (+3.94%)	⬆️
server/tso.go	80.85% <0%> (+4.25%)	⬆️
... and 1 more

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[lhy1024]

/merge

[sre-bot]

/run-all-tests