code: public static void propagateTracingContext(Span span, HttpHeaders headers) {
      Preconditions.checkArgument(span != null, "span should not be null.");
      Preconditions.checkArgument(headers != null, "headers should not be null.");
      if (propagationTextFormat != null && propagationTextFormatSetter != null) {
        if (!span.equals(BlankSpan.INSTANCE)) {
          propagationTextFormat.inject(span.getContext(), headers, propagationTextFormatSetter);
        }
      }
    }
docs: Propagate information of current tracing context. This information will be injected into HTTP
  header.
  
  @param span the span to be propagated.
  @param headers the headers used in propagation.