dashboard: fix a bug that could not handle proxy requests correctly (#5326)

[ti-chi-bot]

This is an automated cherry-pick of #5326

What problem does this PR solve?

Issue Number: Close #5321

What is changed and how does it work?

Only process headers added by PD.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

Fix a bug that could not handle dashboard proxy requests correctly.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• bufferflies
• lhy1024

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[codecov]

Codecov Report

Base: 75.01% // Head: 75.08% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (28dd7fe) compared to base (eea08fb).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 28dd7fe differs from pull request most recent head 67e9cf9. Consider uploading reports for the commit 67e9cf9 to get more accurate results

Additional details and impacted files

@@               Coverage Diff               @@
##           release-5.3    #5357      +/-   ##
===============================================
+ Coverage        75.01%   75.08%   +0.07%     
===============================================
  Files              263      263              
  Lines            27359    27360       +1     
===============================================
+ Hits             20522    20543      +21     
+ Misses            5024     5014      -10     
+ Partials          1813     1803      -10     

Flag	Coverage Δ
unittests	75.08% <100.00%> (+0.07%)	⬆️

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

Impacted Files	Coverage Δ
pkg/dashboard/adapter/redirector.go	91.30% <100.00%> (+6.85%)	⬆️
pkg/cache/lru.go	79.06% <0.00%> (-9.31%)	⬇️
server/region_syncer/client.go	75.19% <0.00%> (-4.66%)	⬇️
pkg/etcdutil/etcdutil.go	84.70% <0.00%> (-3.53%)	⬇️
server/tso/tso.go	67.97% <0.00%> (-2.81%)	⬇️
server/member/member.go	63.10% <0.00%> (-2.14%)	⬇️
client/base_client.go	79.89% <0.00%> (-2.07%)	⬇️
pkg/btree/btree.go	85.78% <0.00%> (-0.97%)	⬇️
server/cluster/coordinator.go	74.36% <0.00%> (-0.43%)	⬇️
server/schedule/operator_controller.go	83.84% <0.00%> (-0.41%)	⬇️
... and 10 more

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[nolouch]

/merge

[ti-chi-bot]

@nolouch: 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: 28dd7fe

[nolouch]

/merge

[ti-chi-bot]

@nolouch: 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: 67e9cf9