add_onnx_metadata support only str value

[Chizkiyahu]

add_onnx_metadata - metadata support only Dict[str, str] please improve docs and errors

metadata = {"foo": "bar",  "float_data":4.2}
add_onnx_metadata(onnx_model, metadata=metadata)

logs

 meta.key, meta.value = k, v
              ^^^^^^^^^^
TypeError: bad argument type for built-in operation