By default, the APM Insight agent captures pre-defined classes of several web components and frameworks. However, in the case of user-defined frameworks, the agent can't identify such classes by default. This is where custom instrumentation comes in handy.
With custom instrumentation, you can track the performance of your methods and classes. Include the specified code at the beginning of your class or method, and the agent will start tracking its performance.
Using custom instrumentation, you can gain deep visibility into your application's performance.
Custom instrumentation in Java
Custom instrumentation in .NET
Custom instrumentation in Ruby