Custom Dimension Mapping: Selectable Extension Fields

When custom dimension mapping is enabled on a dataset, you can now choose exactly which extension-mapping dimensions and metrics to bring in, instead of importing all of them automatically. Open the Customize Fields modal and use the new "Extension Mapping Fields" tab alongside the existing dataset template fields — extension fields appear in the dimension and metric tables with a distinct badge so they're easy to spot.

Your selection is saved with the dataset and respected end-to-end through validation and ingestion. Existing datasets are unchanged: until you open Customize Fields and pick a subset, all mapping fields continue to come through as before. This is especially useful for sources with strict per-dataset dimension limits, where pulling in every mapping field at once can break ingestion.