code: def _update_token(self, request):
          
  
          # Refresh our source credentials.
          self._source_credentials.refresh(request)
  
          body = {
              "delegates": self._delegates,
              "scope": self._target_scopes,
              "lifetime": str(self._lifetime) + "s"
          }
  
          headers = {
              'Content-Type': 'application/json',
          }
  
          # Apply the source credentials authentication info.
          self._source_credentials.apply(headers)
  
          self.token, self.expiry = _make_iam_token_request(
              request=request,
              principal=self._target_principal,
              headers=headers,
              body=body)
docs: Updates credentials with a new access_token representing
          the impersonated account.
  
          Args:
              request (google.auth.transport.requests.Request): Request object
                  to use for refreshing credentials.