We can build a custom extension in APPD using the following three mechanisms

• Using a script:
  You can write a shell script (Linux and Unix-like systems) or batch file (Windows) to report custom metrics every minute to the Standalone Machine Agent. The Standalone Machine Agent passes these metrics on to the Controller.
  For more information, see Build a Monitoring Extension Using Scripts.
• Using Java:
  Your custom metrics might be too complicated to collect using a script. For example, you might need to perform complex calculations or call a third-party API to get the metrics. In this case, you can extend the JavaServersMonitor class to collect the metrics and report them to the Standalone Machine Agent. Your Java program extends the JavaServersMonitor class to provide your custom functionality.
  See Build a Monitoring Extension Using Java.
• Using HTTP:
  If you enable the agent HTTP listener, you can post HTTP requests to the Standalone Machine Agent to send it custom metrics every minute. This is done by starting the Standalone Machine Agent with a Jetty HTTP listener.
  See Standalone Machine Agent HTTP Listener for information on starting the HTTP listener and sending it metrics.