We use the term camunda as a source for all camunda activity instance events. However, there is nothing stopping users having the same term for external events. This makes it difficult to distinguish the source of where a given event came from, particularly in the context of event counting where we would need to know which indices to count. We should prevent event ingestion with this reserved source.

ATs:

• We prevent the ingestion of external events using camunda as the source property. We should probably reject the whole batch if a single event is found violating this, making user retries easier
• Our upgrade notes state this new restriction
• We consider the migration needed for any existing external events that have camunda as a source. It would be enough to explain the indices that need updating on this ticket