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:

• lhy1024
• nolouch

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[codecov]

Codecov Report

Base: 75.32% // Head: 75.21% // Decreases project coverage by -0.10% ⚠️

Coverage data is based on head (f4854c7) compared to base (1f79ab5).
Patch coverage: 66.66% of modified lines in pull request are covered.

❗ Current head f4854c7 differs from pull request most recent head 41f0fbe. Consider uploading reports for the commit 41f0fbe to get more accurate results

Additional details and impacted files

@@               Coverage Diff               @@
##           release-5.1    #5355      +/-   ##
===============================================
- Coverage        75.32%   75.21%   -0.11%     
===============================================
  Files              245      245              
  Lines            24801    24802       +1     
===============================================
- Hits             18681    18655      -26     
- Misses            4491     4518      +27     
  Partials          1629     1629              

Flag	Coverage Δ
unittests	75.21% <66.66%> (-0.11%)	⬇️

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

Impacted Files	Coverage Δ
server/grpc_service.go	48.34% <25.00%> (ø)
pkg/dashboard/adapter/redirector.go	91.30% <100.00%> (+6.85%)	⬆️
server/schedulers/shuffle_hot_region.go	55.10% <0.00%> (-10.21%)	⬇️
server/member/member.go	65.59% <0.00%> (-3.23%)	⬇️
server/tso/allocator_manager.go	77.29% <0.00%> (-3.18%)	⬇️
server/election/leadership.go	83.52% <0.00%> (-2.36%)	⬇️
server/tso/tso.go	75.00% <0.00%> (-2.23%)	⬇️
server/schedule/operator/step.go	67.92% <0.00%> (-1.58%)	⬇️
client/base_client.go	82.98% <0.00%> (-1.55%)	⬇️
server/cluster/coordinator.go	73.22% <0.00%> (-0.72%)	⬇️
... and 8 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: 41f0fbe