Cannot call get_metadata on a DataFrameModel if there is a Config without a metadata attribute

[cbrummitt]

Describe the bug
I have a DataFrameModel with a custom Config. When I tried to define a class method that returns a subset of columns with certain metadata, I found that I needed to either add metadata = None on my custom config or change the config to inherit from BaseConfig. It'd be nice to allow using column-level metadata without having to define schema-level metadata.

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of pandera.
• (optional) I have confirmed this bug exists on the main branch of pandera.

Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

Here's a minimal example that raises AttributeError: type object 'Config' has no attribute 'metadata':

import pandera as pa


class SchemaA(pa.DataFrameModel):
    foo: str = pa.Field(metadata=dict(include=True))
    bar: str = pa.Field()

    @classmethod
    def subset(cls) -> list[str]:
        return [
            column
            for column, metadata in cls.get_metadata()[str(cls)]["columns"].items()
            if metadata is not None and metadata.get("include")
        ]

    class Config:
        strict = True


print(SchemaA.subset())

Expected behavior

If we add metadata = None on Config or make the Config inherit from pandera.api.pandas.model_config.BaseConfig, then the snippet above produces the expected output of ["foo"].

Desktop (please complete the following information):

• Version: pandera 0.19.2