code: def __arrow_ext_serialize__(self) -> bytes:
      metadata = {"freq": self.freq}
      return json.dumps(metadata).encode()
  
imports: from __future__ import annotations
  import json
  from typing import TYPE_CHECKING
  import pyarrow
  from pandas.compat import pa_version_under14p1
  from pandas.core.dtypes.dtypes import IntervalDtype, PeriodDtype
  from pandas.core.arrays.interval import VALID_CLOSED
label_desc: Use image node_id 3 for calling the ArrowPeriodType obj's underlying member method code with example usage: obj.__arrow_ext_serialize__() and returns: json