contrib/net/http: Add new option to RoundTripper to ignore outgoing request

[a2ikm]

Overview

Please add a new option to RoundTripper in contrib/net/http to determine if an outgoing request tracing should be skipped.
This option should work like the following:

import (
    "net/http"
    httptrace "gopkg.in/DataDog/dd-trace-go.v1/contrib/net/http"
)

opt := httptrace.RTWithIgnoreRequest(func(r *http.Request) bool {
    return r.URL.Path == "/ping"
})

client := httptrace.WrapClient(&http.Client{}, opt)

It is like WithIgnoreRequest for ServeMux.

Background

I run a frontend server and a backend service. Both of them have endpoints for healthcheck to manage their container liveness. The frontend's one replies the response from the backend's one as is.
These endpoints are called very frequently. So I want to disable tracing on them so as not to fill up the tracing log screen.

I can disable incoming tracing with WithIgnoreRequest, but can't outgoing tracing now.

[dianashevchenko]

@a2ikm thanks for reaching out 🙌 I will open a PR shortly

[a2ikm]

Thank you!