Update RN Custom Metrics to use Sentry.setMeasurement (#12496)

Co-authored-by: Alex Krawiec <[email protected]>

Author: krystofwoldrich

docs/platforms/react-native/tracing/instrumentation/performance-metrics.mdx

@@ -6,7 +6,10 @@ sidebar_order: 20
 
 Sentry's SDKs support sending performance metrics data to Sentry. These are numeric values attached to transactions that are aggregated and displayed in Sentry.
 
-<PlatformContent includePath="performance/automatic-performance-metrics" />
+The React Native SDK automatically collects the following performance metrics:
+
+- <PlatformLink to="/tracing/instrumentation/automatic-instrumentation/#app-start-instrumentation"> Cold and warm app start time </PlatformLink>
+- <PlatformLink to="/tracing/instrumentation/automatic-instrumentation/#slow-and-frozen-frames"> Slow and frozen frame rendering </PlatformLink>
 
 ## Custom Measurements
 
@@ -20,7 +23,16 @@ To set a performance measurement, you need to supply the following:
 
 Sentry supports adding arbitrary custom units, but we recommend using one of the [supported units listed below](./#supported-measurement-units).
 
-<PlatformContent includePath="performance/custom-performance-metrics" />
+```javascript
+// Record amount of memory used
+Sentry.setMeasurement("memoryUsed", 123, "byte");
+
+// Record time when Footer component renders on page
+Sentry.setMeasurement("ui.footerComponent.render", 1.3, "second");
+
+// Record amount of times localStorage was read
+Sentry.setMeasurement("localStorageRead", 4);
+```
 
 <Include name="custom-measurements-units-disclaimer.mdx" />