add docBlocks
create BBPatch_20

metered_price and metered_setup_price fields added to product_payment table
add metered pricing fields to product configuration page
create MeteredInterface
create admin.product_is_metered api call

checks if product service can handle metered billing.
add  metered payment case when getting prices

getProductPrice returns always 0 price for metered  payment. Client will be billed for usage later.
update bb-update patch_20

create metered_usage table
implement setUsage() when activating order