golang middleware example https://play.golang.org/p/1Rcyps1TRwO

middleware.go

package main

import (
	"fmt"
)

type Service interface {
	Do()
}

type service struct {}

func (s service) Do() {
	fmt.Println("service.Do()")	
}

type metricMiddleware struct {
	name string
	next Service
}

func (m metricMiddleware) Do() {
	fmt.Printf("metricMiddleware.Do(), metric: %s\n", m.name) // the middleware injected
	m.next.Do()
}

type traceMiddleware struct {
	name string
	next Service
}

func (t traceMiddleware) Do() {
	fmt.Printf("traceMiddleware.Do(), trace: %s\n", t.name) // the middleware injected
	t.next.Do()
}
	
func main() {
	var s Service
	s = service{}
	s = metricMiddleware{"request_count", s}
	s = traceMiddleware{"request", s}
	
	s.Do()
}