`Duration` `Debug` for 2.5s rounds inconsistently for `duration` and `duration.as_secs_f64()`

```rust
use std::time::Duration;

fn main() {
    let a = Duration::new(2, 500000000);
    println!("{:.0?}", a);
    println!("{:.0?}s", a.as_secs_f64());
    println!("{:.0?}", Duration::MAX);
}
```

I expected this to print (round to even):
```
2s
2s
18446744073709551616s
```

Instead, this prints:
```
3s
2s
0s
```