public abstract byte[] encode(MutableSpan input);
@Test void errorSpan_JSON_V2() {
    assertThat(new String(encoder.encode(errorSpan), UTF_8))
        .isEqualTo(
            "{\"traceId\":\"dc955a1d4768875d\",\"id\":\"dc955a1d4768875d\",\"kind\":\"CLIENT\",\"localEndpoint\":{\"serviceName\":\"isao01\"},\"tags\":{\"error\":\"\"}}");
  }