@Test void collectorMetrics_spans() throws IOException {
    assertPostSpansV1Success(LOTS_OF_SPANS[0]);
    assertPostSpansV1Success(LOTS_OF_SPANS[1], LOTS_OF_SPANS[2]);
    assertThat(zipkin.collectorMetrics().spans()).isEqualTo(3);
  }
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