Investigate if `ome-zarr-py` Labels class __init__ leads to performance implications

[LucaMarconato]

I noticed that the constructor of the Labels class in the recent ome-zarr-py release calls a function that computes unique values for the biggest scale of labels: https://github.com/ome/ome-zarr-py/blob/025e83da618ec9e74f4b41f05814fce07b029356/ome_zarr/classes/image.py#L1140

I haven't done regression tests on this, but let's keep an eye and see if this means that we are getting a slowdown upon read.

CC @ajkswamy @Tomaz-Vieira @melonora