feat: add metric value unit type

[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 #580 (0173cf8) into master (2d324e8) will increase coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head 0173cf8 differs from pull request most recent head 24dba48. Consider uploading reports for the commit 24dba48 to get more accurate results

@@            Coverage Diff             @@
##           master     #580      +/-   ##
==========================================
+ Coverage   85.76%   85.79%   +0.03%     
==========================================
  Files         227      227              
  Lines        4720     4731      +11     
==========================================
+ Hits         4048     4059      +11     
  Misses        672      672              

Impacted Files	Coverage Δ
...ers/lint_analyzer/metrics/models/metric_value.dart	100.00% <ø> (ø)
...zer/metrics/metrics_list/lines_of_code_metric.dart	100.00% <100.00%> (ø)
...metrics/metrics_list/number_of_methods_metric.dart	100.00% <100.00%> (ø)
...rce_lines_of_code/source_lines_of_code_metric.dart	100.00% <100.00%> (ø)
...analyzers/lint_analyzer/metrics/models/metric.dart	100.00% <100.00%> (ø)
...ers_list/console/lint_console_reporter_helper.dart	88.88% <100.00%> (ø)
...s_list/html/components/report_details_tooltip.dart	100.00% <100.00%> (ø)
...orters/reporters_list/json/lint_json_reporter.dart	100.00% <100.00%> (ø)

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