Response postSpansV1(List<Span> spans) throws IOException {
    byte[] spansInJson = SpanBytesEncoder.JSON_V1.encodeList(spans);
    return client.newCall(new Request.Builder().url(zipkin.httpUrl() + "/api/v1/spans")
      .post(RequestBody.create(MediaType.parse("application/json"), spansInJson))
      .build()).execute();
  }
Normally, a span can be reported twice: for client and server. However, there are bugs that
happened where several updates went to the same span id. {@link ZipkinExtension#collectorMetrics}
can be used to help ensure a span isn't reported more times than expected.
zipkin-junit5/src/test/java/zipkin2/junit5/ZipkinExtensionTest.java