This static member function is optional. It returns the payload size of `body` in bytes not including any chunked transfer encoding. The return value may be zero, to indicate that the message is known to have no payload. The function shall not exit via an exception. When this function is present: * The function shall not fail * A call to [link beast.ref.boost__beast__http__message.payload_size `message::payload_size`] will return the same value as `size`. * A call to [link beast.ref.boost__beast__http__message.prepare_payload `message::prepare_payload`] will remove "chunked" from the Transfer-Encoding field if it appears as the last encoding, and will set the Content-Length field to the returned value. Otherwise, when the function is omitted: * A call to [link beast.ref.boost__beast__http__message.payload_size `message::payload_size`] will return `boost::none`. * A call to [link beast.ref.boost__beast__http__message.prepare_payload `message::prepare_payload`] will erase the Content-Length field, and add "chunked" as the last encoding in the Transfer-Encoding field if it is not already present.