public static Tag<HttpRequest> requestHeader(String headerName) {
    return requestHeader(headerName, headerName);
  }
@Test void requestHeader() {
    when(request.header("User-Agent")).thenReturn("Mozilla/5.0");
    HttpTags.requestHeader("User-Agent").tag(request, span);

    verify(span).tag("User-Agent", "Mozilla/5.0");
  }