[gitlab-org/gitlab] Track usage of the Rapid Diffs opt-in toggle

### ROOT CAUSE The issue involves tracking user opt-in behavior for the Rapid Diffs feature in GitLab merge requests (MRs). Currently, there is no analytics event triggered when a user enables the toggle. The root cause is the absence of tracking code to record this user interaction, making it impossible to measure adoption or usage of the feature. ### CODE FIX To track the opt-in event, we need to: 1. Add an analytics event when the user enables the toggle. 2. Use GitLab's `Tracking` module to record the event with appropriate properties. Here's the fix: ```ruby # app/controllers/projects/merge_requests/rapid_diffs_controller.rb class Projects::MergeRequests::RapidDiffsController < ApplicationController before_action :authenticate_user! def update @project = Project.find(params[:project_id]) @merge_request = MergeRequest.find(params[:merge_request_id]) @enabled = params[:rapid_diffs][:enabled] == 'true' # Update the feature flag for the project (or user, depending on scope) Spree::FeatureFlag::RapidDiffs::FeatureToggleService.call(@project, @enabled) # Track the opt-in event if @enabled Gitlab::Tracking::Analytics.track( 'rapid_diffs_opt_in', current_user.id, project_id: @project.id, merge_request_id: @merge_request.id ) end head :ok end end ``` This code: - Updates the feature flag for the project (or user, adjust as needed). - Triggers an analytics event (`rapid_diffs_opt_in`) when the toggle is enabled. - Includes properties like `user_id`, `project_id`, and `merge_request_id` for context. The event should be defined in GitLab's analytics infrastructure (e.g., in `app/lib/analytics/events/`) to ensure proper data collection.