code: func SpanOrNoopFromContext(ctx context.Context) Span {
  	if s, ok := ctx.Value(spanKey).(Span); ok {
  		return s
  	}
  	return defaultNoopSpan
  }
docs: // SpanOrNoopFromContext retrieves a Zipkin Span from Go's context propagation
  // mechanism if found. If not found, returns a noopSpan.
  // This function typically is used for modules that want to provide existing
  // Zipkin spans with additional data, but can't guarantee that spans are
  // properly propagated. It is preferred to use SpanFromContext() and test for
  // Nil instead of using this function.