Hello @Frederic DDurand ,
welcome to this moderated Azure community forum.
The GetMetadataPropertyValue is the right way to read (user or application) properties and system properties of the EventData message format.
I see you tried to read both [Kafka] and [EventHub] properties. Perhaps this works already but the column output was just not represented.
The Azure portal ASA query editor output is not able to show the outcome of a GetMetadataPropertyValue column. Please use eg. an Azure function logging or an Event Hub to read and display the output instead of struggling with 'null'.
If this does not help, can you also try User as in [User].
Can you also check out this related question, seen in the past?
If this still does not work, it could be the new Kafka input (which is in preview) is still not supported by the GetMetadataPropertyValue statement.
In that case, give feedback in the documentation or provide product feedback.
If the response helped, do "Accept Answer". If it doesn't work, please let us know the progress. All community members with similar issues will benefit by doing so. Your contribution is highly appreciated.