@Test void utf8_sizeInBytes() {
    Utf8Field field = new Utf8Field(1 << 3 | WIRETYPE_LENGTH_DELIMITED);
    assertThat(field.sizeInBytes("12345678"))
      .isEqualTo(0
        + 1 /* tag of string field */ + 1 /* len */ + 8 // 12345678
      );
  }
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