net/mwan3: expand ubus to show when last mwan3track check was done

To know how old the ubus output is, add an age parameter which indicats
how old the check informations on the interface are.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

net/mwan3/files/usr/libexec/rpcd/mwan3

@@ -34,7 +34,8 @@ get_mwan3_status() {
 	local iface="${1}"
 	local iface_select="${2}"
 	local running="0"
-	local pid device
+	local age=0
+	local pid device time_p time_n
 
 	network_get_device device $1
 
@@ -44,7 +45,14 @@ get_mwan3_status() {
 			running="1"
 		fi
 
+		time_p="$(cat "$MWAN3_STATUS_DIR/${iface}/TIME")"
+		[ -z "${time_p}" ] || {
+			time_n="$(date +'%s')"
+			let age=time_n-time_p
+		}
+
 		json_add_object "${iface}"
+		json_add_int age "$age"
 		json_add_string "score" "$(cat "$MWAN3_STATUS_DIR/${iface}/SCORE")"
 		json_add_string "lost" "$(cat "$MWAN3_STATUS_DIR/${iface}/LOST")"
 		json_add_string "turn" "$(cat "$MWAN3_STATUS_DIR/${iface}/TURN")"