@Test void span_write_omitsEmptyEndpoints() {
    SPAN.write(buf, spanBuilder()
      .localEndpoint(Endpoint.newBuilder().build())
      .remoteEndpoint(Endpoint.newBuilder().build())
      .build());

    assertThat(buf.pos())
      .isEqualTo(22);
  }
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