main.go

req, err := http.NewRequest("GET", "http://service_a_envoy:8788/", nil)
if err != nil {
	fmt.Printf("%s", err)
}

req.Header.Add("x-request-id", r.Header.Get("x-request-id"))
req.Header.Add("x-b3-traceid", r.Header.Get("x-b3-traceid"))
req.Header.Add("x-b3-spanid", r.Header.Get("x-b3-spanid"))
req.Header.Add("x-b3-parentspanid", r.Header.Get("x-b3-parentspanid"))
req.Header.Add("x-b3-sampled", r.Header.Get("x-b3-sampled"))
req.Header.Add("x-b3-flags", r.Header.Get("x-b3-flags"))
req.Header.Add("x-ot-span-context", r.Header.Get("x-ot-span-context"))

client := &http.Client{}
resp, err := client.Do(req)

Good for demo but this is not something to be done in production. Better to use openTracing and Jaeger libraries to do this.