reset multi-index to single index (level): coordinate not renamed

[benbovy]

What happened?

Resetting a multi-index to a single level (i.e., a single index) does not rename the remaining level coordinate to the dimension name.

What did you expect to happen?

While it is certainly more consistent not to rename the level coordinate here (since an index can be assigned to a non-dimension coordinate now), it breaks from the old behavior. I think it's better not introduce any breaking change. As discussed elsewhere, we might eventually want to deprecate reset_index in favor of drop_indexes (#6971).

Minimal Complete Verifiable Example

import pandas as pd
import xarray as xr

midx = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=("foo", "bar"))

ds = xr.Dataset(coords={"x": midx})
# <xarray.Dataset>
# Dimensions:  (x: 4)
# Coordinates:
#   * x        (x) object MultiIndex
#   * foo      (x) object 'a' 'a' 'b' 'b'
#   * bar      (x) int64 1 2 1 2
# Data variables:
#     *empty*

rds = ds.reset_index("foo")

# v2022.03.0
#
# <xarray.Dataset>
# Dimensions:  (x: 4)
# Coordinates:
#   * x        (x) int64 1 2 1 2
#     foo      (x) object 'a' 'a' 'b' 'b'
# Data variables:
#     *empty*

# v2022.06.0
# 
# <xarray.Dataset>
# Dimensions:  (x: 4)
# Coordinates:
#     foo      (x) object 'a' 'a' 'b' 'b'
#   * bar      (x) int64 1 2 1 2
# Dimensions without coordinates: x
# Data variables:
#     *empty*

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