Add dice example

[pellared]

Towards #4531

Mostly based on https://opentelemetry.io/docs/instrumentation/python/getting-started/ (but also looked at other languages)

Remarks

I did my best to make the example reasonably close to “real production code” (however, without tests).

I started with

package main

import (
	"io"
	"log"
	"math/rand"
	"net/http"
	"strconv"
)

func main() {
	http.HandleFunc("/rolldice", func(w http.ResponseWriter, r *http.Request) {
		roll := 1 + rand.Intn(6)
		resp := strconv.Itoa(roll) + "\n"
		if _, err := io.WriteString(w, resp); err != nil {
			log.Printf("Write failed: %v\n", err)
		}
	})

	log.Fatal(http.ListenAndServe(":8080", nil))
}

But I needed graceful shutdown (via SIGINT) to properly handle cleanup. Second SIGINT should kill the application.
I also decided to restructure the code and e.g. avoid using globals.
Moreover, I set server's WriteTimeout and ReadTimeout as it is a good practice.

[pellared]

@svrnm @cartermp PTAL

[svrnm]

nice work @pellared

[pellared]

93a69bc is addressing comments from open-telemetry/opentelemetry.io#3310 (review)

[pellared]

@dashpole Can you please take a quick look as I think you already reviewed this code? 😉