I have a 2015 Surly Long Haul Trucker with a Andel RSC6 crank and Shimano UN-55 bottom bracket. Is there a crankset I could replace that with that would be compatible with a single side power meter?
Power meters for square taper BBs are hard to come by, and usually custom made and expensive.
However, fortunately the Surly LHT has an ordinary English threaded bottom bracket. Just swap out the UN-55 for any Hollowtech 2 compatible external BB, such as the Shimano Tiagra BB-RS500 or 105 SM-BBR60. Then, get any hollowtech 2 crank, replace the left side with a power meter, and you're good to go.
In fact, more recent models of the LHT use hollowtech-2-type crankset systems out of the factory.
I know Stages only sells power meters for 105 and above for road, so it might make sense to get a 105 BB and a 105 crankset, coupled with a 105 power meter such as the stages Gen 3 Power L, or 4iiii equivelant.
If you want more gearing range than a 105 crankset provides, you can mix and match with other cranksets that use the Hollowtech 2 interface, of which there are plenty.
While product recommendations are off-topic here due to their tendency to become obsolete, it's fair to say that what you're up against here is that touring triples (24-36-46 or thereabouts) of the type that LHTs come with are pretty far opposite from the type of cranks that get left-crank power meters.
But, what may exist is a 110/74 or 104/64 triple direct-mount spider for a crank of the milieu that get left power cranks made. Sometimes third party companies make such spiders. There are a lot of 104/64 double direct-mount spiders for such cranks, but I don't think there would be a way to hack them into running a triple without running into clearance problems with the crankarm and FD or the ring itself.
You can probably find an older native 104/64 crank with a power meter where you'd be able to replicate your chainring sizes, but the Q and chainline may not be right or at least may not be what you had.
Shimano has made various HT2 triple cranks with Euro trekking type chainring configurations. You could conceivably find a match between one of those and one of the XTR/XT/Saint/DXR power left cranks. HT2 mountain power crank interchangeability is kind of a maze and is again an area where things change, so it would be on you to figure that out. That would probably be the simplest off the shelf way to get something close to what you have. In other words, there are definitely 104/64 HT2 cranks that can take the same gearing as what you have and can also match with extant HT2 power cranks. I believe the stock LHT chainline might be in a bit from what you'd get if you did that though.