Metadata in `nnx.param_field` is not passed to the Param class

When I create a new parameter I can pass extra keyword arguments as metadata:

```python
import dataclasses
from flax.experimental import nnx

foo = nnx.Param(
1.0,
domain = "positive_real",
)

print(foo)
```
```
Param(
value=1.0,
domain='positive_real'
)
```

I would expect that when passing a dictionary of the corresponding kwargs to `nnx.param_field`, they would be passed down to the `Param` class, but they are not:

```python
@nnx.dataclass
class Bar(nnx.Module):
foo: jax.Array = nnx.param_field(1.0, metadata={"domain": "positive_real"})

bar = Bar()
print(bar.variables)
```
```
VariablesMapping{
foo: Param(
value=1.0
)
}
```

However they can be found here: `dataclasses.fields(bar)[0].metadata["domain"]`.

I am just wondering if this is expected or a bug. Either way, I believe it would make sense to have this possibility. I have a quick fix (change the signature of the `nnx_variable_constructor` and actually pass the metadata in `ModuleMeta`) for it and I'd be happy to open a PR if you welcome this change :)

P.S.: another possibility would be to slightly change the `nnx.param_field` function signature: `metadata` becomes `**metadata`, this way one could do something like:
```python
@nnx.dataclass
class Bar(nnx.Module):
foo: jax.Array = nnx.param_field(1.0, domain="positive_real")
```