There are several key metrics that you should monitor when using APM tools:
1. Response time: This measures how long it takes for a request to be processed and a response to be returned.
2. Error rate: This measures the number of errors that occur in the application, such as failed requests or unhandled exceptions.
3. Resource usage: This measures the amount of system resources, such as CPU and memory, that the application is using.
4. Throughput: This measures the number of requests that the application can handle within a certain time period.
5. Latency: This measures the time it takes for data to travel from the client to the server and back.
6. User experience: This measures the overall satisfaction of the users with the application.
7. Business metric: This measures the impact the application has on the business, such as the number of transactions, revenue, conversion rate, etc.
It's important to monitor these metrics over time to detect trends and patterns, and to be able to identify when and where performance issues occur in the application.
Also, it's important to note that depending on the specific requirements of your application, you may need to monitor additional or different metrics.