@Override public Span tag(String key, String value) {
    synchronized (state) {
      state.tag(key, value);
    }
    return this;
  }
@Test void tag() {
    span.tag("foo", "bar");
    span.flush();

    assertThat(spans).flatExtracting(s -> s.tags().entrySet())
      .containsExactly(entry("foo", "bar"));
  }