transport/datagram/failure: allow adding debug data

enjo_lib/comm/transport/datagrams.py

@@ -21,7 +21,7 @@ from ...our_types import (
 @serializable
 class GeneralTransportDatagram:
     _version: int = 1
-    timestamp: int
+    timestamp_ns: int
     sender: InstanceId
     message_type: str
     data: JSON | GeneralTransportMetadata | None
@@ -90,8 +90,16 @@ class FailureDatagram(GeneralTransportDatagram):
 
 @serializable
 class FailureMetadata(GeneralTransportMetadata):
+    """
+    Issues that a specific service has failed.
+
+    This does not mean that the whole stack might be failed as each service may run on its own.
+    But most certainly, if the controller does issue an failure, it likely is the whole stack.
+    """
+
     epoch: EpochId
     reason: HumanFriendlyDesc
+    debug_data: JSON
 
 
 # restart