function _getResponseSizeFromHeaders(headers) {
  if (headers) {
    const contentLength = headers['Content-Length'] || headers['content-length'];

    if (contentLength) {
      return parseInt(contentLength, 10);
    }
  }

  return undefined;
}
Extracts response size from `Content-Length` header when possible

@param headers
@returns The response size in bytes or undefined