Event Metrics are the most flexible way of capturing metric data in your application. As long as it's possible to record a sample every time an event occurs you can use Event Metrics to record multiple related values that can then be analyzed in the Loupe Desktop.
Event Metrics are particularly well suited for multithreaded or stateless applications because they emphasize recording data about each discrete event (like a single database call or web hit) instead of attempting to aggregate data across threads.
For a generalized approach on how to design and implement event metrics for your application's needs, see Developer's Guide - Metrics - Designing Event Metrics.
See a complete example of how you can use event metrics to get performance and usage information of all of the database queries in your application in Developer's Guide - Metrics - Database Query Event Metric Example.