Is a paper with no citations likely to contribute to citation metrics in a way which reduces my job prospects?
No, this is not a problem. Everyone has papers on their CV which have few citations, including older papers. If this were true for all your papers, in particular even all of those which are several years old, this could be an issue - but this does not seem to be a problem at all in your case.  (Note that the distribution of citations vs. papers is somewhat similar for everyone, reflected e.g. in the fact that there is a clear relation between the number of publications and the h-index.)
Moreover, if you really feel later on that a given paper does not represent you well, you can always omit it from your publication record - you are not obliged to list everything you ever published there, in particular if you feel it is not relevant.
So I would go with being a coauthor of the paper, if the others offer coauthorship to you, and you feel it is deserved.

Edit: To follow up on your updated title, a paper with a low number of citations will not have a negative effect on citation metrics: Typical metrics are either the total number of papers, the total number of citations, or (most importantly) the h-index, none of which gets worse if you add little-cited papers.  The only indicator which would be affected is the number of citations per paper, but this is rarely used - and even then, a single little-cited paper will not have a big effect.