Add in_product_marketing_emails experiment

config/feature_flags/experiment/in_product_marketing_emails_experiment_percentage.yml

+---
+name: in_product_marketing_emails_experiment_percentage
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50679
+rollout_issue_url: https://gitlab.com/gitlab-org/growth/team-tasks/-/issues/303
+milestone: "13.9"
+type: experiment
+group: group::activation
+default_enabled: false

lib/gitlab/experimentation.rb

@@ -109,6 +109,9 @@ module Gitlab
       },
       trial_onboarding_issues: {
         tracking_category: 'Growth::Conversion::Experiment::TrialOnboardingIssues'
+      },
+      in_product_marketing_emails: {
+        tracking_category: 'Growth::Activation::Experiment::InProductMarketingEmails'
       }
     }.freeze