[DNM]add example for hardcode metrics list

[nexustar]

What problem does this PR solve?

What is changed and how it works?

Check List

Tests

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

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

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

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

[codecov-commenter]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.48%. Comparing base (c0cacb0) to head (e3cf30e).
⚠️ Report is 31 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/diag/command/root.go	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
- Coverage   30.11%   28.48%   -1.63%     
==========================================
  Files          50       50              
  Lines        4218     4213       -5     
==========================================
- Hits         1270     1200      -70     
- Misses       2778     2845      +67     
+ Partials      170      168       -2     

Flag	Coverage Δ
unittests	28.48% <40.00%> (-1.63%)	⬇️

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[nexustar]

example metrics are dumped from a tiup-playground deployed local cluster, may not precise

[dveeden]

@kaaaaaaang I think this should have the "not-merge" label?