fix: unbundle 2-label metrics

[twoeths]

Motivation

• On network thread, it takes 3% of cpu time for gauge.inc()

Description

• Use single-label metric to improve performance

Closes #426

[codecov-commenter]

Codecov Report

Patch coverage: 3.33% and project coverage change: -0.22 ⚠️

Comparison is base (324c693) 82.96% compared to head (eb89073) 82.74%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
- Coverage   82.96%   82.74%   -0.22%     
==========================================
  Files          48       48              
  Lines       11896    11930      +34     
  Branches     1282     1282              
==========================================
+ Hits         9869     9871       +2     
- Misses       2027     2059      +32     

Impacted Files	Coverage Δ
src/metrics.ts	19.67% <3.33%> (-0.61%)	⬇️

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