Hi Frank,
Thank you for reaching out. If you go to the campaign resource and select “Yes, it is” for question “Is campaign specified in the FROM clause of your query?” you would find segments.geo_target_metro is not in the Segments list. This means you could not use segments.geo_target_metro as a segmentation in your query. You could consider using the geographic_view resource to fetch the geo target metrics.
Thanks and regards,
Xiaoming, Google Ads API Team