optimize(metric): optimize calculation of counter rate

[justadogistaken]

What type of PR is this?

What this PR does / why we need it:

Which issue(s) this PR fixes:

Special notes for your reviewer:

[codecov]

Codecov Report

Attention: 200 lines in your changes are missing coverage. Please review.

Comparison is base (cc764cd) 53.37% compared to head (bc79633) 53.27%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #342      +/-   ##
==========================================
- Coverage   53.37%   53.27%   -0.11%     
==========================================
  Files         431      434       +3     
  Lines       47368    47590     +222     
==========================================
+ Hits        25283    25353      +70     
- Misses      19236    19389     +153     
+ Partials     2849     2848       -1     

Flag	Coverage Δ
unittest	53.27% <18.36%> (-0.11%)	⬇️

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

Files	Coverage Δ
pkg/custom-metric/mock/utils.go	100.00% <100.00%> (ø)
...server/agent/metric/malachite/fetcher_calculate.go	67.69% <67.64%> (-6.23%)	⬇️
pkg/metaserver/agent/metric/malachite/fetcher.go	52.56% <0.00%> (-0.54%)	⬇️
pkg/custom-metric/mock/mock_store.go	0.00% <0.00%> (ø)
pkg/custom-metric/mock/mock_collector.go	0.00% <0.00%> (ø)

... and 13 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.