Added missing information about kafka-pubsub special metadata headers

Signed-off-by: Patrick Assuied <patrick.assuied@elationhealth.com>

daprdocs/content/en/reference/components-reference/supported-pubsub/setup-apache-kafka.md

@@ -468,7 +468,7 @@ Apache Kafka supports the following bulk metadata options:
 
 When invoking the Kafka pub/sub, its possible to provide an optional partition key by using the `metadata` query param in the request url.
 
-The param name is `partitionKey`.
+The param name can either `partitionKey` or `__key` 
 
 Example:
 
@@ -484,7 +484,7 @@ curl -X POST http://localhost:3500/v1.0/publish/myKafka/myTopic?metadata.partiti
 
 ### Message headers
 
-All other metadata key/value pairs (that are not `partitionKey`) are set as headers in the Kafka message. Here is an example setting a `correlationId` for the message.
+All other metadata key/value pairs (that are not `partitionKey` or `__key`) are set as headers in the Kafka message. Here is an example setting a `correlationId` for the message.
 
 ```shell
 curl -X POST http://localhost:3500/v1.0/publish/myKafka/myTopic?metadata.correlationId=myCorrelationID&metadata.partitionKey=key1 \
@@ -495,7 +495,51 @@ curl -X POST http://localhost:3500/v1.0/publish/myKafka/myTopic?metadata.correla
         }
       }'
 ```
+### Kafka Pubsub special message headers received on consumer side
 
+When consuming messages, special message metadata are being automatically passed as headers. These are:
+- `__key`: the message key if applicable
+- `__topic`: the topic for the message
+- `__partition`: the partition number for the message
+- `__offset`: the offset of the message in the partition
+- `__timestamp`: the timestamp for the message
+
+You can access them within the consumer endpoint as follows:
+{{< tabs "Python (FastAPI)" >}}
+
+{{% codetab %}}
+
+```python
+from fastapi import APIRouter, Body, Response, status
+import json
+import sys
+
+app = FastAPI()
+
+router = APIRouter()
+
+
+@router.get('/dapr/subscribe')
+def subscribe():
+    subscriptions = [{'pubsubname': 'pubsub',
+                      'topic': 'my-topic',
+                      'route': 'my_topic_subscriber',
+                      }]
+    return subscriptions
+
+@router.post('/my_topic_subscriber')
+def my_topic_subscriber(
+      key: Annotated[str, Header(alias="__key")],
+      offset: Annotated[int, Header(alias="__offset")],
+      event_data=Body()):
+    print(f"key={key} - offset={offset} - data={event_data}", flush=True)
+      return Response(status_code=status.HTTP_200_OK)
+
+app.include_router(router)
+
+```
+
+{{% /codetab %}}
 ## Receiving message headers with special characters
 
 The consumer application may be required to receive message headers that include special characters, which may cause HTTP protocol validation errors.