Is a paper with no citations likely to contribute to citation metrics in a way which reduces my job prospects? Last year I did some work to help out my friend and his supervisor on a project that they were working on. I wasn't very involved with the project, but the work that I contributed was important for them to be able to find the main results that they were interested in. I asked at the time if they would they be happy to include me as an author on the paper based on this contribution, and they said they would be happy to do that.
I think it's quite likely that the paper might not get very many citations. The most closely related paper to the one they will publish soon was written by my friend's supervisor, and it has only one citation. It was published in 2015, and I think it's in an area that doesn't have so much research interest in general.
I've been wondering what different people think about whether being an author on this work would look better or worse for me, with respect to future positions in academia. I'm hoping to look for a postdoctoral position in the near future, in a different but related field.
I feel like it might look better: I would have more publications and it's showing more of my skills. I could put it on my CV.
Or it might look worse: I know there are different metrics which take into consideration how many citations papers have, and having a paper with no citations might reduce my score on them.
I'm currently at the end of my PhD and I have 6 publications, one has 4 citations and the rest have between 10 and 25. I think at this point it would be reasonable to ask them to include me only as an acknowledgement on the paper, if I decided that was the best option for me.
Any opinions/perspectives on this appreciated.
I appreciate that it's my choice whether I put the paper on my CV or not, if I choose to stay as an author.
Thanks
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.