Create a builder metric from a registered dataset. Source connection and table metadata are derived from the dataset. Requires name, metric_type, builder_config.dataset_id, metric_value_column, and aggregation. User metrics also require dataset_group_id and an eligible secondary Dataset, never the Primary Dataset. The path from Primary Dataset to Metric Dataset must contain a one-to-many step and no many-to-one or many-to-many step. Model validation errors return 422 with errors and error_details (attribute, code, message). Defaults to draft. Service account callers must supply an organization-scoped creator_id.