function getTotalBytes$6(instance) {
	const body = instance.body;


	if (body === null) {
		// body is null
		return 0;
	} else if (isBlob$6(body)) {
		return body.size;
	} else if (Buffer.isBuffer(body)) {
		// body is buffer
		return body.length;
	} else if (body && typeof body.getLengthSync === 'function') {
		// detect form data input from form-data module
		if (body._lengthRetrievers && body._lengthRetrievers.length == 0 || // 1.x
		body.hasKnownLength && body.hasKnownLength()) {
			// 2.x
			return body.getLengthSync();
		}
		return null;
	} else {
		// body is stream
		return null;
	}
}
The Fetch Standard treats this as if "total bytes" is a property on the body.
For us, we have to explicitly get it with a function.

ref: https://fetch.spec.whatwg.org/#concept-body-total-bytes

@param   Body    instance   Instance of Body
@return  Number?            Number of bytes, or null if not possible