Complete 100 line coverage for Metadata mixin

This adds a final spec to cover an intentionally unimplemented method.

Author: corsonknowles

spec/rubocop/cop/rspec/mixin/metadata_spec.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::RSpec::Metadata do
+  describe '#on_metadata' do
+    subject(:on_metadata) do
+      stub_class.new.on_metadata(:symbol, {})
+    end
+
+    let(:stub_class) do
+      Class.new do
+        include RuboCop::Cop::RSpec::Metadata
+      end
+    end
+
+    it { expect { on_metadata }.to raise_error(NotImplementedError) }
+  end
+end