update description and datatype

result/port.yaml

@@ -144,8 +144,13 @@ components:
               x-field-uid: 1
             stopped:
               x-field-uid: 2
-        last_link_state_change_time:
-          description: >-
-            The timestamp of the last link-state change event
-          type: string
+        last_change:
+          description: |-
+            The timestamp indicates the absolute time of the last 
+            link state change of the test port (e.g., up-to-down transition).
+      
+            The value is the timestamp in nanoseconds relative to
+            the Unix Epoch (Jan 1, 1970 00:00:00 UTC).
+          type: integer
+          format: uint64
           x-field-uid: 14