refactor(metaserver): refactor notify init in metaserver

[itonyli]

What type of PR is this?

Refactor

What this PR does / why we need it:

Simplify Notify registration for different metrics through the closure method.

Which issue(s) this PR fixes:

Special notes for your reviewer:

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.20%. Comparing base (5e1814b) to head (0e00bfe).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #596      +/-   ##
==========================================
+ Coverage   56.14%   56.20%   +0.05%     
==========================================
  Files         535      535              
  Lines       49865    49821      -44     
==========================================
+ Hits        27997    28000       +3     
+ Misses      18307    18278      -29     
+ Partials     3561     3543      -18     

Flag	Coverage Δ
unittest	56.20% <100.00%> (+0.05%)	⬆️

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