@Test public void tags() {

  MutableSpan span = new MutableSpan();
  assertThat(span.tagCount()).isZero();
  assertThat(span.tags()).isEmpty();
  span.tag("http.method", "GET");
  assertThat(span.tagCount()).isOne();
  assertThat(span.tags()).containsExactly(
      entry("http.method", "GET")
  );
  span.tag("error", "500");
  assertThat(span.tagCount()).isEqualTo(2);
  assertThat(span.tags()).containsExactly(
      entry("http.method", "GET"),
      entry("error", "500")
  );
  span.forEachTag((t, v) -> v.equals("GET") ? v : null);
  assertThat(span.tagCount()).isOne();
  assertThat(span.tags()).containsExactly(
      entry("http.method", "GET")
  );
}