In the WMF EventStreams API, if a Kafka message has a key, it is included in the outgoing event at meta.key (not part of the original event schema).

However, the key is being serialized as a Javascript Buffer. E.g.

"key": {
   "type": "Buffer",
   "data": [
     123,
     34,
     119,
     105,
     107,
     105,
     95,
     105,
     100,
     34,
     58,
     34,
     99,
     111,
     109,
     109,
     111,
     110,
     115,
     119,
     105,
     107,
     105,
     34,
     44,
     34,
     112,
     97,
     103,
     101,
     95,
     105,
     100,
     34,
     58,
     49,
     48,
     49,
     51,
     49,
     51,
     53,
     50,
     54,
     125
   ]

I think the way to do this will be to properly deserialize the key in the first place here.

However, there is no guarantee that the key will be a JSON string...that is just a convention we use at WMF. We should add safeguards too, just in case.

An example of how this is exposed to external users of EventStreams (like Wikimedia Enterprise) is in T373644.