code: func (h *Handler) hostCredentials(w http.ResponseWriter, r *http.Request, p httprouter.Params) (interface{}, error) {
  	var req auth.RegisterUsingTokenRequest
  	if err := httplib.ReadJSON(r, &req); err != nil {
  		return nil, trace.Wrap(err)
  	}
  
  	authClient := h.cfg.ProxyClient
  	packedKeys, err := authClient.RegisterUsingToken(req)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  
  	return packedKeys, nil
  }
docs: // hostCredentials sends a registration token and metadata to the Auth Server
  // and gets back SSH and TLS certificates.