code: def _fetch_certs(request, certs_url):
      
      response = request(certs_url, method='GET')
  
      if response.status != http_client.OK:
          raise exceptions.TransportError(
              'Could not fetch certificates at {}'.format(certs_url))
  
      return json.loads(response.data.decode('utf-8'))
docs: Fetches certificates.
  
      Google-style cerificate endpoints return JSON in the format of
      ``{'key id': 'x509 certificate'}``.
  
      Args:
          request (google.auth.transport.Request): The object used to make
              HTTP requests.
          certs_url (str): The certificate endpoint URL.
  
      Returns:
          Mapping[str, str]: A mapping of public key ID to x.509 certificate
              data.