Here is the old R package https://github.com/jwijffels/RMETAR It uses C code and was removed from CRAN "as basic C error stopping installation on OS X was not corrected within 6 weeks.". RMETAR only accessed the current metar from NOAA:
It's nice that it's called RMETAR therefore "rmetar" is still available.
The decoding part of RMETAR used C code, see https://github.com/jwijffels/RMETAR/blob/master/pkg/R/decodemetar.R It connects to mdsplib (METAR Decoder Software Package Library - http://mdsplib.sourceforge.net/)
The licence is "License: LGPL-2.1"
I wonder whether one needs C code for decoding metar (surely there must be other decoders?), and if one does, I don't know how to use C code. Then, if I did, I wondered how one can re-use code from an old package?
Another possibility is that somewhere there's an archive of all decoded METAR. Now I know there are all METAR on the mesonet website but maybe the decoded version is somewhere else.