Delta 
Delta is an HTTP shadow proxy server that sits between clients and your server(s) to enable "shadow requests".
It's actually just a Go port of Kage. You can consult the documentation of Kage for reasons why this software matters ;)
Usage
package main
import (
"github.com/kentaro/delta"
"log"
"net/http"
"time"
)
func main() {
server := delta.NewServer("0.0.0.0", 8484)
server.AddMasterBackend("production", "127.0.0.1", 8080)
server.AddBackend("testing", "127.0.0.1", 8081)
server.OnSelectBackend(func(req *http.Request) []string {
if req.Method == "GET" {
return []string{"production", "testing"}
} else {
return []string{"production"}
}
})
server.OnMungeHeader(func(backend string, header *http.Header) {
if backend == "testing" {
header.Add("X-Delta-Sandbox", "1")
}
})
server.OnBackendFinished(func(responses map[string]*delta.Response) {
for backend, response := range responses {
log.Printf("%s [%d ms]: %s", backend, (response.Elapsed / time.Millisecond), response.Data)
}
})
server.Run()
}