@Test void fixed64_sizeInBytes() {
    Fixed64Field field = new Fixed64Field(1 << 3 | WIRETYPE_FIXED64);
    assertThat(field.sizeInBytes(Long.MIN_VALUE))
      .isEqualTo(9);
  }
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