code: func (c *Client) ExtendWebSession(user string, prevSessionID string) (services.WebSession, error) {
  	out, err := c.PostJSON(
  		c.Endpoint("users", user, "web", "sessions"),
  		createWebSessionReq{
  			PrevSessionID: prevSessionID,
  		},
  	)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	return services.GetWebSessionMarshaler().UnmarshalWebSession(out.Bytes())
  }
docs: // ExtendWebSession creates a new web session for a user based on another
  // valid web session