id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
792639470,MDU6SXNzdWU3OTI2Mzk0NzA=,4839,Coordinate attributes are dropped when interpolating datasets,14371165,closed,0,,,3,2021-01-23T20:05:33Z,2021-04-27T07:00:08Z,2021-04-27T07:00:08Z,MEMBER,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

Please be concise with code posted. See guidelines below on how to provide a good bug report:

- Craft Minimal Bug Reports: http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports
- Minimal Complete Verifiable Examples: https://stackoverflow.com/help/mcve

Bug reports that follow these guidelines are easier to diagnose, and so are often handled much more quickly.
-->

**What happened**:
When I was interpolating datasets I noticed that the coordinate variables disappeared.

**What you expected to happen**:
Coordinate attributes should be retained just like variables are.

**Minimal Complete Verifiable Example**:

```python
import numpy as np
import xarray as xr

names = np.core.defchararray.add(""long_variable_name"", np.arange(0, 2).astype(str))
coords = dict(time=np.array([0, 1]))
data_vars = dict()
for v in names:
    data_vars[v] = xr.Variable(
        ""time"", np.array([0, 1], dtype=int), attrs=dict(unit=""kg"")
    )
ds1 = xr.Dataset(data_vars=data_vars, coords=coords)
ds1.attrs = {
    k: 2 for k in np.core.defchararray.add(""attr_"", np.arange(0, 3).astype(str))
}
ds1.time.attrs.update(unit=""s"")

# Print time:
ds1.time
Out[115]: 
<xarray.DataArray 'time' (time: 2)>
array([0, 1])
Coordinates:
  * time     (time) int32 0 1
Attributes:
    unit:     s

# Interpolate:
ds1 = ds1.interp(
    time=np.array([0, 0.5, 1, 2]),
    assume_sorted=True,
    method=""linear"",
    kwargs=dict(fill_value=""extrapolate""),
)

# Print interpolated time, units are lost:
ds1.time
Out[117]: 
<xarray.DataArray 'time' (time: 4)>
array([0. , 0.5, 1. , 2. ])
Coordinates:
  * time     (time) float64 0.0 0.5 1.0 2.
```

**Anything else we need to know?**:

**Environment**:

<details><summary>Output of <tt>xr.show_versions()</tt></summary>

<!-- Paste the output here xr.show_versions() here -->
xr.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.8.5 (default, Sep  3 2020, 21:29:08) [MSC v.1916 64 bit (AMD64)]
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
libhdf5: 1.10.4
libnetcdf: None

xarray: 0.16.2
pandas: 1.2.0
numpy: 1.17.5
scipy: 1.4.1
netCDF4: None
pydap: None
h5netcdf: None
h5py: 2.10.0
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.3.2
dask: 2020.12.0
distributed: 2020.12.0
matplotlib: 3.3.2
cartopy: None
seaborn: 0.11.1
numbagg: None
pint: None
setuptools: 51.1.2.post20210112
pip: 20.3.3
conda: 4.9.2
pytest: 6.2.1
IPython: 7.19.0
sphinx: 3.4.3

</details>
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/4839/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue