`intake_xarray` does not lazy read metadata from files

The entries powered by `intake_xarray` driver does not lazy read metadata from the files.

```python
# %%
import intake
import xarray as xr

ds = xr.Dataset(
{
"test_var": [0],
},
attrs={"xarray_metadata": "The metadata in the xarray file"},
)
ds.to_netcdf("test_metadata.nc")
ds.to_zarr("test_metadata.zarr", mode="w")
# %%
catalog_content = """sources:
netcdf:
driver: netcdf
args:
urlpath: '{{ CATALOG_DIR }}/test_metadata.nc'
metadata:
catalog_metadata: The metadata in the catalog entry
zarr_intake_xarray:
description: zarr archive read by intake_xarray
driver: zarr
args:
urlpath: '{{ CATALOG_DIR }}/test_metadata.zarr'
metadata:
catalog_metadata: The metadata in the catalog entry
zarr_intake:
description: zarr archive read by intake
driver: zarr_cat
args:
urlpath: '{{ CATALOG_DIR }}/test_metadata.zarr'
metadata:
catalog_metadata: The metadata in the catalog entry
"""

with open("catalog.yml", "w") as f:
f.write(catalog_content)

cat = intake.open_catalog("catalog.yml")
print(f"{cat.netcdf.metadata = }")
print(f"{cat.zarr_intake_xarray.metadata = }")
print(f"{cat.zarr_intake.metadata = }")
```
As you see from the output, the metadata from the entry powered by `intake` driver has the field from the `zarr` file:
```
cat.netcdf.metadata = {'catalog_metadata': 'The metadata in the catalog entry'}
cat.zarr_intake_xarray.metadata = {'catalog_metadata': 'The metadata in the catalog entry'}
cat.zarr_intake.metadata = {'catalog_metadata': 'The metadata in the catalog entry', 'xarray_metadata': 'The metadata in the xarray file'}
```
However, after reading the files, the metadata is complete:
```python
cat.netcdf.read()
cat.zarr_intake_xarray.read()

print(f"Netcdf metadata after reading: {cat.netcdf.metadata}")
print(f"Zarr metadata after reading: {cat.zarr_intake_xarray.metadata}")
```
Output:
```
Netcdf metadata after reading: {'catalog_metadata': 'The metadata in the catalog entry', 'dims': {'test_var': 1}, 'data_vars': {}, 'coords': ('test_var',), 'xarray_metadata': 'The metadata in the xarray file'}
Zarr metadata after reading: {'catalog_metadata': 'The metadata in the catalog entry', 'dims': {'test_var': 1}, 'data_vars': {}, 'coords': ('test_var',), 'xarray_metadata': 'The metadata in the xarray file'}
```
OS: Windows 10
python 3.11.5
intake 0.7.0
intake_xarray 0.7.0
xarray 2023.8.0
zarr 2.16.1