attrs are not preserved in serializations through Arrow or Parquet

[jpivarski]

Version of Awkward Array

HEAD

Description and code to reproduce

>>> array = ak.Array([1, 2, 3], attrs={"one": 1})
>>> array.attrs
{'one': 1}

but

>>> ak.from_arrow(ak.to_arrow(array)).attrs
{}

and

>>> ak.to_parquet(array, "/tmp/array.parquet")
<pyarrow._parquet.FileMetaData object at 0x6ffe4b371350>
  created_by: parquet-cpp-arrow version 14.0.2
  num_columns: 1
  num_rows: 3
  num_row_groups: 1
  format_version: 2.6
  serialized_size: 0
>>> ak.from_parquet("/tmp/array.parquet").attrs
{}

Did this work at one point and stop working? If so, weren't there tests?

Pickle works:

>>> pickle.loads(pickle.dumps(array)).attrs
{'one': 1}

Metadata handling changed considerably in #3127; did it work before that? (#3127 would have changed ak.to_parquet via ak.to_arrow_table, but not ak.to_arrow.) This relies on the metadata-handling mechanisms, but extensionarray=True is on by default:

>>> ak.from_arrow(ak.to_arrow(array, extensionarray=True)).attrs
{}

>>> ak.to_parquet(array, "/tmp/array.parquet", extensionarray=True)
<pyarrow._parquet.FileMetaData object at 0x6ffe495bad90>
  created_by: parquet-cpp-arrow version 14.0.2
  num_columns: 1
  num_rows: 3
  num_row_groups: 1
  format_version: 2.6
  serialized_size: 0
>>> ak.from_parquet("/tmp/array.parquet").attrs
{}