client.go
func Tracer(newTracer NewTracer) ClientParam {
	return func(c *Client) error {
		c.newTracer = newTracer
		return nil
	}
}
func Tracer(newTracer NewTracer) ClientParam {
	return func(c *Client) error {
		c.newTracer = newTracer
		return nil
	}
}
// Tracer sets a request tracer constructor