refactor: cleanup metric documentation

[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
[ ] Other, please explain:

What changes did you make? (Give an overview)

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

[codecov]

Codecov Report

Merging #547 (fd18031) into master (39011e5) will decrease coverage by 0.11%.
The diff coverage is 46.66%.

❗ Current head fd18031 differs from pull request most recent head 9cad7e4. Consider uploading reports for the commit 9cad7e4 to get more accurate results

@@            Coverage Diff             @@
##           master     #547      +/-   ##
==========================================
- Coverage   84.95%   84.83%   -0.12%     
==========================================
  Files         219      219              
  Lines        4472     4483      +11     
==========================================
+ Hits         3799     3803       +4     
- Misses        673      680       +7     

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)
...r/metrics/metrics_list/weight_of_class_metric.dart	100.00% <ø> (ø)
...zers/lint_analyzer/metrics/models/file_metric.dart	0.00% <ø> (ø)
... and 3 more

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