@Test void wireType_unsupported() {
    for (int unsupported : List.of(3, 4, 6)) {
      try {
        Field.wireType(1 << 3 | unsupported, 2);
        failBecauseExceptionWasNotThrown(IllegalArgumentException.class);
      } catch (IllegalArgumentException e) {
        assertThat(e)
          .hasMessage("Malformed: invalid wireType " + unsupported + " 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