Lists the custom dimensions and custom metrics REGISTERED on the GA4 property linked to a site (GA4 Admin > Custom definitions) - the event parameters and user properties that property collects on top of the standard GA4 fields, such as a source_format or target_path sent with an internal_link_click event. Each entry carries the exact apiName (e.g. customEvent:source_format) that query_ga4_custom_dimensions takes. Call this FIRST: an event parameter that is not registered cannot be reported on at all, and GA4 answers an unregistered name with an empty report rather than an error, so a name must never be guessed. Returns a notConfigured result (not an error) when Google Analytics is not connected or no GA4 property is linked to the site in the GSC Wizard app.