You could probably implement a custom processor like "JsonToAttributes" very easily, iterate through a JSON document and add each field/value as an attribute to the flow file, or do it with a Groovy script in ExecuteScript. The reason it doesn't exist is that in the general case you should be careful about adding a significant amount of attributes because those are held in memory, so if we provided this processor people would start using it to adding 100s of attributes to their flow files which could lead to poor performance and memory issues.
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