@Test void boolean_sizeInBytes() {
    BooleanField field = new BooleanField(1 << 3 | WIRETYPE_VARINT);

    assertThat(field.sizeInBytes(false))
      .isZero();
    assertThat(field.sizeInBytes(true))
      .isEqualTo(0
        + 1 /* tag of varint field */ + 1 // size of 1
      );
  }
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