public abstract byte[] encode(MutableSpan input);
@Test void span_minimum_JSON_V2() {
    MutableSpan span = new MutableSpan();
    span.traceId("7180c278b62e8f6a216a2aea45d08fc9");
    span.id("5b4185666d50f68b");

    assertThat(new String(encoder.encode(span), UTF_8))
        .isEqualTo(
            "{\"traceId\":\"7180c278b62e8f6a216a2aea45d08fc9\",\"id\":\"5b4185666d50f68b\"}");
  }