"coordinates" metadata must be passed through for CMIP7 usage

[ceblanton]

Is your feature request related to a problem? Please describe.
fremor requires coordinates variable attribute to determine/confirm its MIP variable status.

This is proper (in the history files)

                tas:long_name = "Near-Surface Air Temperature" ;
                tas:units = "K" ;
                tas:missing_value = 1.e+20f ;
                tas:_FillValue = 1.e+20f ;
                tas:coordinates = "height2m" ;
                tas:cell_methods = "time: mean" ;
                tas:cell_measures = "area: area" ;
                tas:standard_name = "air_temperature" ;

But this pp output is deficient:

		tas:standard_name = "air_temperature" ;
		tas:long_name = "Near-Surface Air Temperature" ;
		tas:units = "K" ;
		tas:_FillValue = 1.e+20f ;
		tas:missing_value = 1.e+20f ;
		tas:cell_methods = "time: mean" ;
		tas:cell_measures = "area: area" ;
		tas:interp_method = "conserve_order2" ;

Describe the solution you'd like
coordinates variable attribute must be passed in

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