It's "demonstrator" in The UK or "product demonstrator" .

I can’t think of a specific word for this. You could say “the sample guy” or “the sample lady” and people would know what you mean, but I wouldn’t use those phrases to the the sampler’s face because they could sound a bit rude. If I were talking to the sampler himself/herself, I might say “a food demonstrator” or something like that.

“sample worker” would be a formal-sounding title. Colloquially I might say “the person offering samples” or “the person giving samples”.

You have it. They are called a “product sampler” or “product demonstrator.”