@Test public void accessorScansTags() {

  MutableSpan span = new MutableSpan();
  span.tag("http.method", "GET");
  span.tag("error", "500");
  span.tag("http.path", "/api");
  assertThat(span.tag("error")).isEqualTo("500");
  assertThat(span.tag("whoops")).isNull();
}