I'm not sure I quite understand the question. CommonMetaData class you mentioned is not a member of RSearch.dll COM library. And for all I know RSearch worksheet function or namesake COM library cannot retrieve instrument descriptions or "long names". It can only retrieve instrument codes. However, what I guess you might be looking for is the instrument description you can see in autosuggest drop-down as you type in text in Eikon command/search bar. I'm afraid there's no public API available through Eikon, which will give you exactly that. However you might want to look into fields TR.InstrumentDescription and/or TR.AssetCategory, which you can retrieve using DEX2 COM library given an instrument code. To quickly try it use the following example formula in Eikon Excel:
=TR("TRI.N","TR.AssetCategory;TR.InstrumentDescription")

Thanks a lot. It works.