in error analysis, refactor error rate metric into function

[imatiach-msft]

Description

In erroranalysis package, refactor the error rate metric computation into a function, to make it more similar to other metrics.

Checklist

• [ ] I have added screenshots above for all UI changes.
• [ ] I have added e2e tests for all UI changes.
• [ ] Documentation was updated if it was needed.

[codecov-commenter]

Codecov Report

Merging #1742 (d93ceb5) into main (6d46af4) will increase coverage by 0.00%. The diff coverage is 84.61%.

@@           Coverage Diff           @@
##             main    #1742   +/-   ##
=======================================
  Coverage   88.54%   88.54%           
=======================================
  Files          96       96           
  Lines        5481     5490    +9     
=======================================
+ Hits         4853     4861    +8     
- Misses        628      629    +1     

Flag	Coverage Δ
unittests	88.54% <84.61%> (+<0.01%)	:arrow_up:

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

Impacted Files	Coverage Δ
erroranalysis/erroranalysis/_internal/metrics.py	88.67% <77.77%> (-2.23%)	:arrow_down:
...is/erroranalysis/_internal/surrogate_error_tree.py	88.17% <100.00%> (+0.31%)	:arrow_up:

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