etcdutil: support multi backends client

[lhy1024]

What problem does this PR solve?

Issue Number: Close #6042

What is changed and how does it work?

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• bufferflies
• rleungx

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[lhy1024]

cc @binshi-bing @rleungx

[codecov]

Codecov Report

Base: 74.90% // Head: 74.90% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (d8741ad) compared to base (94d05f0).
Patch coverage: 90.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6046   +/-   ##
=======================================
  Coverage   74.90%   74.90%           
=======================================
  Files         365      366    +1     
  Lines       36552    36562   +10     
=======================================
+ Hits        27378    27388   +10     
+ Misses       6786     6783    -3     
- Partials     2388     2391    +3     

Flag	Coverage Δ
unittests	74.90% <90.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
pkg/mcs/resource_manager/server/server.go	48.05% <ø> (ø)
pkg/utils/etcdutil/etcdutil.go	80.48% <78.94%> (-0.14%)	⬇️
client/base_client.go	83.17% <100.00%> (ø)
pkg/core/region_tree.go	95.56% <100.00%> (ø)
pkg/utils/etcdutil/testutil.go	100.00% <100.00%> (ø)
server/schedule/waiting_operator.go	97.36% <100.00%> (ø)
server/server.go	74.38% <100.00%> (-0.26%)	⬇️
pkg/errs/errs.go	75.00% <0.00%> (-25.00%)	⬇️
pkg/storage/endpoint/rule.go	83.33% <0.00%> (-11.12%)	⬇️
pkg/utils/metricutil/metricutil.go	82.75% <0.00%> (-10.35%)	⬇️
... and 23 more

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[rleungx]

LGTM except for the unresolved comment.

[bufferflies]

BTW, Is there some releationship between the timeouts, like defaultEtcdClientTimeout ,DefaultDialTimeout,DefaultRequestTimeout?

[lhy1024]

/merge

[ti-chi-bot]

@lhy1024: 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: d8741ad