@Test void span_write_startsWithFieldInListOfSpans() {
    SPAN.write(buf, spanBuilder().build());

    assertThat(bytes).startsWith(
      0b00001010 /* span key */, 20 /* bytes for length of the 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