void assertRoundTrip(Span span) {
    SPAN.write(buf, span);

    assertThat(SPAN.read(ReadBuffer.wrap(bytes)))
      .isEqualTo(span);
  }
A map entry is an embedded messages: one for field the key and one for the value
zipkin/src/test/java/zipkin2/internal/Proto3ZipkinFieldsTest.java