Add new experiment class to house experiment logic

- This adds an experiment class and moves all inline logic into the
experiment implementation.

app/experiments/new_project_readme_content_experiment.rb

+# frozen_string_literal: true
+
+class NewProjectReadmeContentExperiment < ApplicationExperiment # rubocop:disable Gitlab/NamespacedClass
+  TEMPLATE_PATH = Rails.root.join('app', 'experiments', 'templates', 'new_project_readme_content')
+
+  def run_with(project, variant: nil)
+    @project = project
+    record!
+    run(variant)
+  end
+
+  def control_behavior
+    template('readme_basic.md')
+  end
+
+  def advanced_behavior
+    template('readme_advanced.md')
+  end
+
+  private
+
+  def template(name)
+    ERB.new(File.read(TEMPLATE_PATH.join("#{name}.tt")), trim_mode: '<>').result(binding)
+  end
+end

app/services/projects/create_service/templates/readme_advanced.md.tt → app/experiments/templates/new_project_readme_content/readme_advanced.md.tt

@@ -2,6 +2,7 @@
 
 <%= @project.description %>
 
+
 ## Getting started
 
 To make it easy for you to get started with GitLab, here's a list of recommended next steps.

app/services/projects/create_service.rb

@@ -3,7 +3,6 @@
 module Projects
   class CreateService < BaseService
     include ValidatesClassificationLabel
-    TEMPLATE_PATH = Rails.root.join('app', 'services', 'projects', 'create_service', 'templates')
 
     def initialize(user, params)
       @current_user = user
@@ -152,21 +151,12 @@ module Projects
         branch_name: @default_branch.presence || @project.default_branch_or_main,
         commit_message: 'Initial commit',
         file_path: 'README.md',
-        file_content: experiment(:new_project_readme_content, namespace: @project.namespace) do |e|
-          e.control { template('readme_basic.md') }
-          e.try(:advanced) { template('readme_advanced.md') }
-          e.record!
-          e.run
-        end
+        file_content: experiment(:new_project_readme_content, namespace: @project.namespace).run_with(@project)
       }
 
       Files::CreateService.new(@project, current_user, commit_attrs).execute
     end
 
-    def template(name)
-      ERB.new(File.read(TEMPLATE_PATH.join("#{name}.tt")), trim_mode: '<>').result(binding)
-    end
-
     def skip_wiki?
       !@project.feature_available?(:wiki, current_user) || @skip_wiki
     end

spec/experiments/new_project_readme_content_experiment_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe NewProjectReadmeContentExperiment, :experiment do
+  subject { described_class.new(namespace: project.namespace) }
+
+  let(:project) { create(:project, name: 'Experimental', description: 'An experiment project') }
+
+  it "renders the basic template" do
+    expect(subject.run_with(project)).to eq(<<~MARKDOWN.strip)
+      # Experimental
+
+      An experiment project
+    MARKDOWN
+  end
+
+  it "renders the advanced template" do
+    expect(subject.run_with(project, variant: :advanced)).to include(<<~MARKDOWN.strip)
+      # Experimental
+
+      An experiment project
+
+      ## Getting started
+    MARKDOWN
+  end
+end