contrib/net/http: make roundTripper propagation (inject, extract) optional

[rangelrealecb]

I need to make an http call to a vendor, and I would like to trace this call (and its parameters), but using the current roundTripper would "leak" the trace_id / parent_id as headers, as they are inject automatically on the call.

I propose adding an option (true by default):

func RTWithPropagation(propagation bool) RoundTripperOption {
	return func(cfg *roundTripperConfig) {
		cfg.propagation = propagation
	}
}

and check this before injecting

        if rt.cfg.propagation {
	    err = tracer.Inject(span.Context(), tracer.HTTPHeadersCarrier(req.Header))
            ...
        }

Fixes #1166

[ajgajg1134]

Thanks for submitting a change! Overall this is looking good just one change requested :)