Backed by Stripe's search API, which indexes with a short delay after a subscription's metadata changes. Don't rely on this method finding a subscription immediately after you write the matching metadata; if you need a strict read-after-write guarantee, look the subscription up through your own storage adapter instead.
Returns only the first match. If more than one subscription could share the same key/value pair, use a more specific value, or a combination you control uniquely, such as prefixing with your own namespace.