Bugs on Rhubarb

Over the course of the summer I’ve noticed bugs on my rhubarb plant. At least I think they’re bugs. They look a lot like ball bearings, seem to do no damage (at least to the rhubarb leaves, which are poisonous), and typically collect near the base of the leaf where it meets the stalk.

Upon closer inspection they are either ball bearings with tiny legs, or not ball bearings at all.

Do you have any idea what they are?