update prometheus config when scale in

[Yujie-Xie]

What problem does this PR solve?

#2327
#2375

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

Release notes:

NONE

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 55.96%. Comparing base (9e11d0e) to head (684f9d9).

❗ Current head 684f9d9 differs from pull request most recent head e9e296e. Consider uploading reports for the commit e9e296e to get more accurate results

Files	Patch %	Lines
pkg/cluster/manager/scale_in.go	48.57%	14 Missing and 4 partials ⚠️
pkg/cluster/manager/destroy.go	52.63%	7 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2387      +/-   ##
==========================================
- Coverage   56.07%   55.96%   -0.11%     
==========================================
  Files         329      329              
  Lines       35040    35082      +42     
==========================================
- Hits        19646    19632      -14     
- Misses      13056    13100      +44     
- Partials     2338     2350      +12     

Flag	Coverage Δ
cluster	45.20% <50.00%> (-0.12%)	⬇️
dm	25.54% <31.48%> (-0.03%)	⬇️
playground	15.78% <ø> (-0.01%)	⬇️
tiup	33.59% <ø> (-0.05%)	⬇️
unittest	22.20% <0.00%> (-0.03%)	⬇️

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