contrib/net/http: configurable client ip recording in NewServeMux without environment variable

[eyasy1217]

Hello DataDog Team,

I am currently using the contrib/net/http package from the dd-trace-go library and leveraging the DD_TRACE_CLIENT_IP_ENABLED environment variable to record client IP addresses.

I propose enhancing the NewServeMux function to allow direct configuration for client IP recording, something like:

tracingMux := httptrace.NewServeMux(
    httptrace.EnableClientIP(),
)

I thought this method more intuitive and flexible for setting client IP records.

[Julio-Guerra]

Hello,

It makes sense indeed.
Would you be up to opening a PR with this addition?

[eyasy1217]

👌
But I can only come up with the following ideas😓

1. Add an EnableClientIP field to http.ServeConfig.

2. Determine this within http.TraceAndServe and tag the client IP.
   e.g.

var ipTags map[string]string
if cfg.EnableClientIP {
    ipTags, _ = httpsec.ClientIPTags(r.Header, true, r.RemoteAddr)
}
for k, v := range ipTags {
    opts = append(opts, tracer.Tag(k, v))
}

However, the same thing is being done within httptrace.StartRequestSpan😓