public abstract byte[] encode(MutableSpan input);
@Test void span_JSON_V2() {
    assertThat(new String(encoder.encode(clientSpan), UTF_8))
        .isEqualTo(
            "{\"traceId\":\"7180c278b62e8f6a216a2aea45d08fc9\",\"parentId\":\"6b221d5bc9e6496c\",\"id\":\"5b4185666d50f68b\",\"kind\":\"CLIENT\",\"name\":\"get\",\"timestamp\":1472470996199000,\"duration\":207000,\"localEndpoint\":{\"serviceName\":\"frontend\",\"ipv4\":\"127.0.0.1\"},\"remoteEndpoint\":{\"serviceName\":\"backend\",\"ipv4\":\"192.168.99.101\",\"port\":9000},\"annotations\":[{\"timestamp\":1472470996238000,\"value\":\"foo\"},{\"timestamp\":1472470996403000,\"value\":\"bar\"}],\"tags\":{\"clnt/finagle.version\":\"6.45.0\",\"http.path\":\"/api\"}}");
  }