A profiler is an agent (an application) that encapsulates a profiling API (such as the .NET profiling API). This agent then registers in the runtime environment (for example, the CLR) as a profiler, when running the application.

The runtime environment then calls the agent when code is loaded, and the agent orchestrates the code using a byte-code injection. This process is transparent to your application and does not require any modification of your code in any way.

In case you want to gather additional information, some APM tools provide APIs that can be used to capture and store data for custom metrics. However, this requires a change in the code of your applications.

On the Azure platform, business metrics will be using Azure Application Insights and New Relic's third-party solution (New Relic APM).