Renamed Custom Performance Measurement to Custom Performance Metric (#5383)

Renamed Custom Performance Measurement to Custom Performance Metric

Author: edwardgou-sentry

src/docs/product/sentry-basics/metrics/index.mdx

@@ -25,6 +25,6 @@ _Indexed_ events are a sub-set of your processed events, and this sub-set is cre
 
 <!-- TO DO: TALK ABOUT DIFFERENT USE CASES HERE -->
 
-## Custom Performance Measurements
+## Custom Performance Metrics
 
 Custom performance metrics on transactions allow you to define metrics that are important to your application and send them to Sentry. Currently, you can set these in the [JavaScript SDK (version `7.0.0` or higher)](/platforms/javascript/performance/instrumentation/performance-metrics/).