add custom problem detector plugin

config/custom-plugin-monitor.json

@@ -0,0 +1,32 @@
+{
+  "plugin": "custom",
+  "pluginConfig": {
+    "invoke_interval": "30s",
+    "timeout": "5s",
+    "max_output_length": 80,
+    "concurrency": 3
+  },
+  "source": "ntp-custom-plugin-monitor",
+  "conditions": [
+    {
+      "type": "NTPProblem",
+      "reason": "NTPIsUp",
+      "message": "ntp service is up"
+    }
+  ],
+  "rules": [
+    {
+      "type": "temporary",
+      "reason": "NTPIsDown",
+      "path": "./config/plugin/check_ntp.sh",
+      "timeout": "3s"
+    },
+    {
+      "type": "permanent",
+      "condition": "NTPProblem",
+      "reason": "NTPIsDown",
+      "path": "./config/plugin/check_ntp.sh",
+      "timeout": "3s"
+    }
+  ]
+}

config/plugin/check_ntp.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+
+# NOTE: THIS NTP SERVICE CHECK SCRIPT ASSUME THAT NTP SERVICE IS RUNNING UNDER SYSTEMD.
+#       THIS IS JUST AN EXAMPLE. YOU CAN WRITE YOUR OWN NODE PROBLEM PLUGIN ON DEMAND.
+
+OK=0
+NONOK=1
+UNKNOWN=2
+
+which systemctl >/dev/null
+if [ $? -ne 0 ]; then
+    echo "Systemd is not supported"
+    exit $UNKNOWN
+fi
+
+systemctl status ntp.service | grep 'Active:' | grep -q running
+if [ $? -ne 0 ]; then
+    echo "NTP service is not running"
+    exit $NONOK
+fi
+
+echo "NTP service is running"
+exit $OK