Let's start again with the general definition: an application metric provides you with an insight on a lower level. You get answers to questions such as: How well does the application handle the workload? What happens in the application under the hood and why?

To gather this information, you must track application logic, monitor database connection requests, and all the application's calls to store and retrieve data. Furthermore, you should also cover the use of dependent services by the application (for example cache, service bus, authorization/authentication, and so on). For a complete picture, you should also collect application framework metrics such as ASP.NET performance counters or CLR performance counters (only .NET framework), details about exceptions triggered by the application, details about application locks, and used threads.