refactor(pool): introduce pool.Manager

[Reasno]

[codecov[bot]]

Codecov Report

Merging #251 (1eaa424) into master (5d8b5d1) will increase coverage by 0.24%. The diff coverage is 90.80%.

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
+ Coverage   81.12%   81.36%   +0.24%     
==========================================
  Files          99      100       +1     
  Lines        5482     5484       +2     
==========================================
+ Hits         4447     4462      +15     
+ Misses        845      831      -14     
- Partials      190      191       +1     

Impacted Files	Coverage Δ
observability/metrics.go	100.00% <ø> (ø)
observability/observability.go	100.00% <ø> (ø)
control/pool/manager.go	88.67% <88.67%> (ø)
control/pool/worker.go	90.00% <90.00%> (ø)
control/pool/dependency.go	100.00% <100.00%> (+33.33%)	:arrow_up:
control/pool/pool.go	100.00% <100.00%> (ø)
ctxmeta/ctxmeta.go	93.10% <0.00%> (+0.44%)	:arrow_up:

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