metrics: remove useless "metric" field in pd.json

[okJiang]

What problem does this PR solve?

Issue Number: Close #8381

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.21%. Comparing base (bdbe73e) to head (b541c23).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8382      +/-   ##
==========================================
- Coverage   77.32%   77.21%   -0.12%     
==========================================
  Files         470      470              
  Lines       61687    61689       +2     
==========================================
- Hits        47701    47634      -67     
- Misses      10405    10471      +66     
- Partials     3581     3584       +3     

Flag	Coverage Δ
unittests	77.21% <ø> (-0.12%)	⬇️

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

[okJiang]

/cc @HuSharp @rleungx

[rleungx]

BTW, what is the metrics used for?

[okJiang]

BTW, what is the metrics used for?

I tried to figure out what it does, but I couldn't find any information about it online.

[HuSharp]

Does the grafana's official documentation explain what this field does?

[HuSharp]

Oh, got it #8382 (comment)

[okJiang]

Can you give me an approve?

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HuSharp, rleungx

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-10 07:51:09.608606033 +0000 UTC m=+430366.843840145: ☑️ agreed by rleungx.
• 2024-07-12 02:02:38.145122246 +0000 UTC m=+582255.380356359: ☑️ agreed by HuSharp.

[ti-chi-bot]

@okJiang: Your PR was out of date, I have automatically updated it for you.

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[ti-chi-bot]

@okJiang: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-integration-realcluster-test	b8cc22b	link	false	/test pull-integration-realcluster-test

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[okJiang]

/test pull-integration-realcluster-test