Update create-pool-extensions.md

Add support for application health extension.

Author: lchenmsft

articles/batch/create-pool-extensions.md

@@ -37,6 +37,7 @@ The following extensions can currently be installed when creating a Batch pool:
 - [Microsoft Antimalware extension for Windows](/azure/virtual-machines/extensions/iaas-antimalware-windows)
 - [Azure Monitor agent for Linux](/azure/azure-monitor/agents/azure-monitor-agent-manage)
 - [Azure Monitor agent for Windows](/azure/azure-monitor/agents/azure-monitor-agent-manage)
+- [Application Health extension](azure/virtual-machines/extensions/health-extension)
 
 You can request support for other publishers and/or extension types by opening a support request.
 
@@ -223,6 +224,12 @@ If Key Vault extension is configured incorrectly, the compute node might be in a
 - [Azure Key Vault extension for Linux](/azure/virtual-machines/extensions/key-vault-linux)
 - [Azure Key Vault extension for Windows](/azure/virtual-machines/extensions/key-vault-windows)
 
+## Application Health extension consideration
+
+Batch Node Agent running on the node always starts a http server that returns the health status of the agent. This http server listens on local IP address 127.0.0.1 and port 29879. It always return a 200 status but with the response body being either healthy or unhealty, any other response (or lack of) is considered "unknown" status.
+
+You can setup your own health server that listens on a different port. Optionally, your health server can query Batch Node Agent server and combine your health signal to generate a composite health result.
+
 ## Next steps
 
 - Learn about various ways to [copy applications and data to pool nodes](batch-applications-to-pool-nodes.md).