code: func (t *Tracer) ExtractHTTPHeaders(r *http.Request) (tracing.Span, context.Context) {
  	// Deserialize tracing context into request.
  	wireContext, _ := t.tracer.Extract(
  		opentracing.HTTPHeaders,
  		opentracing.HTTPHeadersCarrier(r.Header),
  	)
  
  	span := t.tracer.StartSpan("HTTP", ext.RPCServerOption(wireContext))
  	ctx := opentracing.ContextWithSpan(r.Context(), span)
  	return span, ctx
  }
docs: // ExtractHTTPHeaders reads the HTTP headers to derive incoming context.