[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: 66.67% and project coverage change: -1.62 ⚠️

Comparison is base (c0cacb0) 30.11% compared to head (1fccce4) 28.49%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
- Coverage   30.11%   28.49%   -1.62%     
==========================================
  Files          50       50              
  Lines        4218     4212       -6     
==========================================
- Hits         1270     1200      -70     
- Misses       2778     2844      +66     
+ Partials      170      168       -2     

Flag	Coverage Δ
unittests	28.49% <66.67%> (-1.62%)	⬇️

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

Impacted Files	Coverage Δ
cmd/diag/command/root.go	20.29% <66.67%> (+0.57%)	⬆️

... and 2 files with indirect coverage changes

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[nexustar]

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