feat: add custom_metric for dynamic batching

[JoanFM]

Goals:

• resolves #ISSUE-NUMBER

• ...

• ...

• ...

• ...

• check and update documentation. See guide and ask the team.

[codecov]

Codecov Report

Attention: Patch coverage is 5.88235% with 32 lines in your changes missing coverage. Please review.

Project coverage is 50.91%. Comparing base (09f61da) to head (148f40a).
Report is 22 commits behind head on master.

Files with missing lines	Patch %	Lines
jina/serve/runtimes/worker/batch_queue.py	6.25%	30 Missing ⚠️
jina/serve/executors/decorators.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6189      +/-   ##
==========================================
- Coverage   55.74%   50.91%   -4.83%     
==========================================
  Files         150      150              
  Lines       14144    14215      +71     
==========================================
- Hits         7884     7237     -647     
- Misses       6260     6978     +718     

Flag	Coverage Δ
jina	50.91% <5.88%> (-4.83%)	⬇️

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