Add history size and continue-as-new suggestion (#178)

temporal/api/history/v1/message.proto

@@ -171,6 +171,13 @@ message WorkflowTaskStartedEventAttributes {
     string identity = 2;
     // TODO: ? Appears unused?
     string request_id = 3;
+    // True if this workflow should continue-as-new soon because its history size (in
+    // either event count or bytes) is getting large.
+    bool suggest_continue_as_new = 4;
+    // Total history size in bytes, which the workflow might use to decide when to
+    // continue-as-new regardless of the suggestion. Note that history event count is
+    // just the event id of this event, so we don't include it explicitly here.
+    int64 history_size_bytes = 5;
 }
 
 message WorkflowTaskCompletedEventAttributes {