add check TCP retransmission (#463)

handler/checker/tasks/observer/network/TCP-retransmission.yaml

@@ -0,0 +1,18 @@
+info: 'Check TCP retransmission. From https://github.com/oceanbase/obdiag/issues/348'
+task:
+  - steps:
+    - type: ssh
+      ssh: 'if command -v tsar &>/dev/null; then     echo "exist"; fi'
+      result:
+        set_value: tsar_exist
+        verify: '[ "exist" == "${tsar_exist}" ]'
+        err_msg: 'tsar is not installed. we can not check tcp retransmission.'
+    - type: ssh
+      ssh: "tsar --check --tcp -s retran | awk -F '=' '{print $2}'"
+      result:
+        set_value: tcp_retransmission
+        verify_type: max
+        verify: 10
+        err_msg: 'tcp retransmission is too high, over 10%.'
+
+