Docs for RN metrics (#15646)

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## DESCRIBE YOUR PR

This PR introduces the "Metrics" docs section for React Native docs.
Comes in combination with this PR that introduces Metrics to RN:
getsentry/sentry-react-native#5402

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## EXTRA RESOURCES

- [Sentry Docs contributor guide](https://docs.sentry.io/contributing/)

Author: alwx

docs/platforms/react-native/metrics/index.mdx

@@ -0,0 +1,30 @@
+---
+title: Set Up Metrics
+sidebar_title: Metrics
+description: "Metrics allow you to send, view and query counters, gauges and measurements from your Sentry-configured apps to track application health and drill down into related traces, logs, and errors."
+sidebar_order: 5760
+beta: true
+---
+
+With Sentry Metrics, you can send counters, gauges, distributions, and sets from your applications to Sentry. Once in Sentry, these metrics can be viewed alongside relevant errors, and searched using their individual attributes.
+
+<Alert>This feature is currently in open beta.</Alert>
+
+## Requirements
+
+<PlatformContent includePath="metrics/requirements" />
+
+## Usage
+
+<PlatformContent includePath="metrics/usage" platform="javascript" />
+
+## Options
+
+<PlatformContent includePath="metrics/options" platform="javascript" />
+
+## Default Attributes
+
+<PlatformContent
+  includePath="metrics/default-attributes"
+  platform="javascript"
+/>

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

@@ -0,0 +1 @@
+<VersionRequirement product="Metrics for React Native" sdk="Sentry React Native SDK" minVersion="7.8.0" />

src/middleware.ts

@@ -1770,10 +1770,6 @@ const USER_DOCS_REDIRECTS: Redirect[] = [
     from: '/platforms/ruby/metrics/',
     to: '/concepts/key-terms/tracing/span-metrics/',
   },
-  {
-    from: '/platforms/react-native/metrics/',
-    to: '/concepts/key-terms/tracing/span-metrics/',
-  },
   {
     from: '/platforms/java/metrics/',
     to: '/concepts/key-terms/tracing/span-metrics/',