fix(RN): RN metrics configure snippet (#10891)

Author: kahest

platform-includes/metrics/configure/react-native.mdx

@@ -0,0 +1,8 @@
+```JavaScript
+Sentry.init({
+  dsn: '___PUBLIC_DSN___',
+  integrations: [
+    Sentry.metrics.metricsAggregatorIntegration(),
+  ],
+});
+```