client: remote var leader which is the same as forwardedHost

[CabinfeverB]

What problem does this PR solve?

Issue Number: ref #4399

What is changed and how does it work?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

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[codecov]

Codecov Report

Merging #7965 (2e68de2) into master (955d30a) will increase coverage by 0.05%.
Report is 77 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7965      +/-   ##
==========================================
+ Coverage   73.48%   73.54%   +0.05%     
==========================================
  Files         436      436              
  Lines       48376    48375       -1     
==========================================
+ Hits        35550    35576      +26     
+ Misses       9768     9743      -25     
+ Partials     3058     3056       -2     

Flag	Coverage Δ
unittests	73.54% <100.00%> (+0.05%)	⬆️

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[ti-chi-bot]

PR needs rebase.

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