@Test void span_write_kind() {
    SPAN.write(buf, spanBuilder().kind(Span.Kind.PRODUCER).build());
    assertThat(bytes)
      .contains(0b0100000, atIndex(22)) // (field_number << 3) | wire_type = 4 << 3 | 0
      .contains(0b0000011, atIndex(23)); // producer's index is 3
  }
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