Metrics shipped with http client cannot be augmented with custom attributes

[zailic]

Description

At the moment it seems there's no way to add custom attributes to the provided metrics within instrumented http client. In the current implementation the metric attributes are added through a Labeler component which is designed to live within context objects tree.
However, looking at the current implementation I'm spotting two issues:

opentelemetry-go-contrib/instrumentation/net/http/otelhttp/transport.go

Lines 146 to 156 in ee76330

	if t.clientTrace != nil {
	ctx = httptrace.WithClientTrace(ctx, t.clientTrace(ctx))
	}
	labeler := &Labeler{}
	ctx = injectLabeler(ctx, labeler)
	r = r.Clone(ctx) // According to RoundTripper spec, we shouldn't modify the origin request.
	// use a body wrapper to determine the request size

1. Line 151 - a new Labeler is created instead of getting from context object (in order not to lose the potential attributes previously created and carried through the context)
2. Line 152 - the injectLabeler method cannot be used outside the otelhttp module so actually this makes impossbile the adding of the Labeler component in the high levels of context object.

Environment

• otelhttp version: latest

Expected behavior

This seems like an easy fix:

• obtain the Labeler via LabelerFromContext method:
  

  opentelemetry-go-contrib/instrumentation/net/http/otelhttp/labeler.go

  Lines 55 to 65 in ee76330

  	// LabelerFromContext retrieves a Labeler instance from the provided context if
  	// one is available. If no Labeler was found in the provided context a new, empty
  	// Labeler is returned and the second return value is false. In this case it is
  	// safe to use the Labeler but any attributes added to it will not be used.
  	func LabelerFromContext(ctx context.Context) (*Labeler, bool) {
  	l, ok := ctx.Value(lablelerContextKey).(*Labeler)
  	if !ok {
  	l = &Labeler{}
  	}
  	return l, ok
  	}

• make injectLabeler public to be invokable outside otelhttp module

After the changes above will be made the following code sample it will be possible:

client := otelhttp.DefaultClient
labeler, _ := otelhttp.LabelerFromContext(ctx)
labeler.Add(
	AppClientID(123),
	AppCredentialID(456),
)
ctx = otelhttp.InjectLabeler(ctx, labeler)
req, _ := http.NewRequestWithContext(ctx, "GET", *url, nil)
res, err := client.Do(req)
if err != nil {
	log.Error(err)
}
defer res.Body.Close()