refactor: prepare for complex metric values

[dkrutskikh]

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix
[ ] New rule
[ ] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[X] Other, please explain:

In some cases we need to calculate complex values by a metric. More details in future PRs.

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?

[github-actions]

Dart Code Metrics unused files report of dart_code_metrics. ✅

Summary

• Scanned package folders: bin, example, lib
• No unused files found! ✅

[github-actions]

Dart Code Metrics analyze report of dart_code_metrics. ✅

Summary

• Scanned folders: bin, example, lib, test

• Total scanned files: 393

• Total lines of source code: 6971

• Total classes: 261

• Average Cyclomatic Number per line of code: 0.35 / 1

• Average Source Lines of Code per method: 6

• Total tech debt: 1142.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #698 (fbfa91c) into master (b0ddf09) will not change coverage.
The diff coverage is 68.75%.

@@           Coverage Diff           @@
##           master     #698   +/-   ##
=======================================
  Coverage   86.57%   86.57%           
=======================================
  Files         257      257           
  Lines        5490     5490           
=======================================
  Hits         4753     4753           
  Misses        737      737           

Impacted Files	Coverage Δ
...matic_complexity/cyclomatic_complexity_metric.dart	100.00% <ø> (ø)
...s_list/halstead_volume/halstead_volume_metric.dart	100.00% <ø> (ø)
...zer/metrics/metrics_list/lines_of_code_metric.dart	100.00% <ø> (ø)
...ics/metrics_list/maintainability_index_metric.dart	100.00% <ø> (ø)
...um_nesting_level/maximum_nesting_level_metric.dart	100.00% <ø> (ø)
...metrics/metrics_list/number_of_methods_metric.dart	100.00% <ø> (ø)
...rics/metrics_list/number_of_parameters_metric.dart	100.00% <ø> (ø)
...rce_lines_of_code/source_lines_of_code_metric.dart	100.00% <ø> (ø)
...ics_list/technical_debt/technical_debt_metric.dart	100.00% <ø> (ø)
...r/metrics/metrics_list/weight_of_class_metric.dart	100.00% <ø> (ø)
... and 15 more

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