By using Azure Application Gateway, you can monitor resources, as follows:

• Backend health: The health of the individual servers in the backend pools can be monitored using the Azure portal, PowerShell, and CLI. An aggregated health summary can be found in the performance diagnostic logs as well. The backend health report shows the output of the application gateway health probe to the backend instances. When probing is not successful and the backend cannot receive traffic, it's considered unhealthy.
• Metrics: Metrics is a feature for certain Azure resources, whereby you can view performance counters in the portal. Azure Application Gateway provides seven metrics so that we can view performance counters: Current connections, Healthy host count, Unhealthy host count, Response Status, Total Requests, Failed Requests, and Throughput.

• Logs: Azure provides different kinds of logs so that we can manage and troubleshoot application gateways. All the logs can be extracted from Azure Blob Storage and viewed in different tools, such as Azure Monitor Logs, Excel, and Power BI. The following types of logs are supported:
  • Activity log: All operations that are submitted to your Azure subscription are displayed in the Azure activity logs (formerly known as operational logs and audit logs). These logs can be viewed from the Azure portal, and they are collected by default.
  • Performance log: This log reflects how the application gateway is performing. It is collected every 60 seconds, and it captures the performance information for each instance, including throughput in bytes, failed request count, total requests served, total requests served, and healthy and unhealthy backend instance count.
  • Firewall log: In cases where the application gateway is configured with the web application firewall, this log can be viewed to display the requests that are logged through either Detection or Prevention mode.
  • Access log: You can use this log to view application gateway access patterns and analyze important information. It is collected every 300 seconds. This information includes the caller's IP, response latency, requested URL, return code, and bytes in and out. This log contains one record per instance of the application gateway. 

Logs can be stored using one of the following three options:

• Storage account: When logs need to be stored for a longer duration, storage accounts are the best solution. When they are stored in a storage account, they can be reviewed when needed.
• Event Hubs: Using Event Hubs, you can integrate the logs with other security information and event management (SIEM) tools to get alerts about your resources.
• Azure Monitor Logs: Azure Monitor Logs is the best solution for real-time monitoring of your application or for looking at trends.

Some basic monitoring can also be done from the overview blade of the application gateway. There, you can find the sum total requests, sum failed requests, sum throughput, and more.