@Test void fieldNumber_malformed() {
    try {
      Field.fieldNumber(0, 2);
      failBecauseExceptionWasNotThrown(IllegalArgumentException.class);
    } catch (IllegalArgumentException e) {
      assertThat(e)
        .hasMessage("Malformed: fieldNumber was zero at byte 2");
    }
  }
Shows we can reliably look at a byte zero to tell if we are decoding proto3 repeated fields.
zipkin/src/test/java/zipkin2/internal/Proto3FieldsTest.java