code: func ExtractHTTP(r *http.Request) propagation.Extractor {
  	return func() (*model.SpanContext, error) {
  		var (
  			traceIDHeader      = r.Header.Get(TraceID)
  			spanIDHeader       = r.Header.Get(SpanID)
  			parentSpanIDHeader = r.Header.Get(ParentSpanID)
  			sampledHeader      = r.Header.Get(Sampled)
  			flagsHeader        = r.Header.Get(Flags)
  		)
  
  		return ParseHeaders(
  			traceIDHeader, spanIDHeader, parentSpanIDHeader, sampledHeader,
  			flagsHeader,
  		)
  	}
  }
docs: // ExtractHTTP will extract a span.Context from the HTTP Request if found in
  // B3 header format.