Assuming you get everything opened and closed properly without having the spring explode at you, do you have the right grease for clockwork in your house? Stuff that does not attract dust and does not get sticky over time? Do you know the right points to apply it to? So that timing is unaffected?
As an extreme example, I had some smart guy use some oil he considered a good idea on the bass mechanics of an accordion. Cue forward a few decades, and it's very hard to clean literally several hundreds of parts from sticky films rendering the instrument unresponsive and unreliable.
A metronome is comparatively coarse clockwork compared to a watch and the case has a lot of space so it's unlikely to be constructed in a way where all parts explode at you unless you don't open it as intended. But the spring has to take a considerable amount of energy it has to give off as sound again, so if you do mess it up, it will likely eject its parts with some insistence when the spring departs its mounting.
At any rate, a clock maker will know how to deal with a squeaky spring regarding both the kind of grease to use and where to apply it (and probably more importantly what not to use and where not to apply it).
That knowledge and the respective skills is most of what you am paying him for, not the actual work and material, both of which will be mostly trivial.