Function register_product_metrics 

pub fn register_product_metrics<I>(
    descriptors: I,
) -> ProductMetricResult<Vec<ProductMetricHandle>>
where
    I: IntoIterator<Item = MetricDescriptor>,

Registers multiple product-owned metric families in order.

The batch is all-or-nothing: when any descriptor fails, the families this batch already registered are rolled back so fixing the failure and retrying does not trip over DuplicateRegistration.