Align with join='override' may update index coordinate metadata

What happened?

It seems that align(*, join="override") may have affected and still may affect the metadata of index coordinate data in an incorrect way. See the MCV example below.

cf. @keewis' original https://github.com/pydata/xarray/pull/6857#discussion_r934425142.

What did you expect to happen?

Index coordinate metadata unaffected by alignment (i.e., metadata is passed through object -> aligned object for each object), like for align with other join methods.

Minimal Complete Verifiable Example

import xarray as xr


ds1 = xr.Dataset(coords={"x": ("x", [1, 2, 3], {"foo": 1})})
ds2 = xr.Dataset(coords={"x": ("x", [1, 2, 3], {"bar": 2})})


aligned1, aligned2 = xr.align(ds1, ds2, join="override")

aligned1.x.attrs
# v2022.03.0 -> {'foo': 1}
# v2022.06.0 -> {'foo': 1, 'bar': 2}
# PR #6857 -> {'foo': 1}
# expected -> {'foo': 1}

aligned2.x.attrs
# v2022.03.0 -> {}
# v2022.06.0 -> {'foo': 1, 'bar': 2}
# PR #6857 -> {'foo': 1, 'bar': 2}
# expected -> {'bar': 2}


aligned11, aligned22 = xr.align(ds1, ds2, join="inner")

aligned11.x.attrs
# {'foo': 1}

aligned22.x.attrs
# {'bar': 2}

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

No response

Anything else we need to know?

No response

Environment

Start by running the minimal example with xr.align(..., join="override") and compare its coordinate attributes with the documented inner-join behavior. Trace the align implementation and existing alignment tests, then add a regression test showing that each aligned object's index coordinate metadata is preserved; done means aligned1.x keeps {'foo': 1} and aligned2.x keeps {'bar': 2}.