[Fix rubocop#12291] Fix a false positive for Metrics/ClassLength

Fixes rubocop#12291.

This PR fixes a false positive for `Metrics/ClassLength`
when a class with a singleton class definition.

changelog/fix_a_false_positive_for_metrics_class_length.md

@@ -0,0 +1 @@
+* [#12291](https://github.com/rubocop/rubocop/issues/12291): Fix a false positive for `Metrics/ClassLength` when a class with a singleton class definition. ([@koic][])

lib/rubocop/cop/metrics/class_length.rb

@@ -42,7 +42,12 @@ class ClassLength < Base
         def on_class(node)
           check_code_length(node)
         end
-        alias on_sclass on_class
+
+        def on_sclass(node)
+          return if node.each_ancestor(:class).any?
+
+          on_class(node)
+        end
 
         def on_casgn(node)
           parent = node.parent

spec/rubocop/cop/metrics/class_length_spec.rb

@@ -17,6 +17,22 @@ class Test
     RUBY
   end
 
+  it 'registers an offense when a class with a singleton class definition has more than 5 lines' do
+    expect_offense(<<~RUBY)
+      class Test
+      ^^^^^^^^^^ Class has too many lines. [8/5]
+        class << self
+          a = 1
+          a = 2
+          a = 3
+          a = 4
+          a = 5
+          a = 6
+        end
+      end
+    RUBY
+  end
+
   it 'reports the correct beginning and end lines' do
     offenses = expect_offense(<<~RUBY)
       class Test