The generic CTR metric "Clickthrough per page visit" (established in T394841) depends on pageview ID:
WITH per_page_interactions AS ( SELECT IF(experiment.assigned = 'control', 'control', 'treatment') AS variation, experiment.subject_id, performer.pageview_id, CAST(SUM(IF(action = 'click', 1, 0)) > 0 AS INT) AS clicked_through FROM {table} WHERE action IN('impression', 'click') AND {where_boilerplate} GROUP BY 1, 2, 3 ) SELECT variation, subject_id, AVG(clicked_through) AS outcome FROM per_page_interactions GROUP BY 1, 2
In order for teams to use it in their experiments, we need the performer_pageview_id contextual attribute collected in product_metrics.web_base stream.