Howdy folks, i've implemented a feature flag with an experiment for a specific product on my ecommerce store. i'm using GA4/BigQuery as the source of my metrics. i'm trying to figure out the best metric to use to measure the performance of this experiment. should I create a custom event in GA4 (e.g. product_specific_purchase) to use as the source for measuring product specific purchases? or is there some way to use the existing GA4 purchase event with a dimension/filter instead?