docs(js): Add metrics page

docs/platforms/javascript/common/metrics/index.mdx

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+---
+title: Set Up Metrics
+sidebar_title: Metrics
+description: "Metrics allow you to send, view and query counters, gauges and measurements sent from your applications within Sentry."
+sidebar_order: 5755
+---
+
+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 limited beta. Please reach out on [GitHub](https://github.com/getsentry/sentry-javascript/discussions/18055) if you have feedback or questions. Features in beta are still in-progress and may have bugs. We recognize the irony.
+</Alert>
+
+## Requirements
+
+<PlatformContent includePath="metrics/requirements" />
+
+## Setup
+
+<PlatformContent includePath="metrics/setup" />
+
+## Usage
+
+<PlatformContent includePath="metrics/usage" />
+
+## Options
+
+<PlatformContent includePath="metrics/options" />
+
+## Default Attributes
+
+<PlatformContent includePath="metrics/default-attributes" />
+

platform-includes/metrics/default-attributes/javascript.mdx

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+By default the SDK will attach the following attributes to a metric:
+
+1. `sentry.environment`: The environment set in the SDK if defined.
+2. `sentry.release`: The release set in the SDK if defined.
+3. `sentry.sdk.name`: The name of the SDK that sent the metric
+4. `sentry.sdk.version`: The version of the SDK that sent the metric
+
+### User Attributes
+
+The SDK will optionally attach user information as attributes (guarded by `sendDefaultPii`):
+
+1. `user.id`
+2. `user.name`
+3. `user.email`
+
+
+### Browser Attributes
+
+The SDK will optionally attach browser information as attributes:
+
+1. `browser.name`
+2. `browser.version`
+3. `sentry.replay_id`: The replay id of the replay that was active when the metric was collected.

platform-includes/metrics/options/javascript.mdx

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+The Sentry JavaScript SDK provides several options to configure how metrics are captured and sent to Sentry.
+
+### Filtering and Modifying Metrics
+
+Use the `beforeSendMetric` callback to filter or modify metrics before they're sent to Sentry. This is useful for:
+
+- Removing sensitive data from metric attributes
+- Dropping metrics you don't want to send
+- Adding or modifying attributes
+
+The callback receives a metric object and must return either a modified metric or `null` to drop it.
+
+```javascript
+Sentry.init({
+  // ...
+  beforeSendMetric: (metric) => {
+    // Drop metrics with specific attributes
+    if (metric.attributes?.dropMe === true) {
+      return null;
+    }
+
+    // Modify metric attributes
+    metric.tags = {
+      ...metric.tags,
+      processed: true,
+    };
+
+    return metric;
+  },
+});
+```
+
+### Disabling Metrics
+
+If you want to disable metrics collection entirely, you can do so by disabling the `_experimental.enableMetrics` flag:
+
+```javascript
+Sentry.init({
+  dsn: "___PUBLIC_DSN___",
+  _experiments: {
+    enable_metrics: false,
+  }
+});
+```
+
+
+### Flushing Metrics
+
+By default, metrics are buffered and flushed depending on buffer size and time. If you need to manually flush metrics before the automatic interval, you can use the `flush` method:
+
+```javascript
+await Sentry.flush();
+```
+
+This will flush all pending metrics and events to Sentry.
+

platform-includes/metrics/requirements/javascript.mdx

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+Metrics are supported in all Sentry JavaScript SDKs version `10.22.0` and above.

platform-includes/metrics/setup/javascript.mdx

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+Metrics are gated by an experimental option, `_experiments.enableMetrics` (will not be required in future versions of the SDK).
+
+```javascript
+Sentry.init({
+  dsn: "___PUBLIC_DSN___",
+  _experiments: {
+    enableMetrics: true,
+  },
+});
+```
+

platform-includes/metrics/usage/javascript.mdx

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+Once the feature is enabled and the SDK is initialized, you can send metrics using the `Sentry.metrics` APIs.
+
+The `metrics` namespace exposes three methods that you can use to capture different types of metric information: `count`, `gauge` and `distribution`.
+
+### Counter
+
+Use `count` to track an incrementing value, such as the number of times a button was clicked or a function was called.
+
+```javascript
+Sentry.metrics.count("button_click", 1);
+```
+
+### Gauge
+
+Use `gauge` to track a value that can go up and down, such as the current memory usage or the number of items in a queue.
+
+```javascript
+Sentry.metrics.gauge("queue_depth", 42);
+```
+
+### Distribution
+
+Use `distribution` to track the distribution of a value, such as the response time of a request.
+
+```javascript
+Sentry.metrics.distribution("response_time", 187.5);
+```
+
+### Adding Attributes
+
+You can also pass additional attributes to any of the metric methods via the `attributes` option. Attributes allow you to filter and group metrics.
+
+```javascript
+Sentry.metrics.count(
+  "button_click",
+  1,
+  {
+    tags: {
+      browser: "Firefox",
+      app_version: "1.0.0",
+    },
+  }
+);
+```
+
+### Specifying Units
+
+For `gauge` and `distribution` metrics, you can specify a unit using the `unit` option. This helps Sentry display the metric value in a human-readable format.
+
+```javascript
+Sentry.metrics.distribution(
+  "response_time",
+  187.5,
+  {
+    unit: "millisecond",
+  }
+);
+
+Sentry.metrics.gauge(
+  "memory_usage",
+  1024,
+  {
+    unit: "byte",
+  }
+);
+```

redirects.js

@@ -1080,14 +1080,6 @@ const userDocsRedirects = [
     source: '/platforms/javascript/tracing/trace-propagation/:path*',
     destination: '/platforms/javascript/tracing/distributed-tracing/:path*',
   },
-  {
-    source: '/platforms/javascript/metrics/:path*',
-    destination: '/platforms/javascript/tracing/span-metrics/:path*',
-  },
-  {
-    source: '/platforms/javascript/guides/:guide/metrics/:path*',
-    destination: '/platforms/javascript/guides/:guide/tracing/span-metrics/:path*',
-  },
   {
     source: '/platforms/javascript/tracing/instrumentation/performance-metrics/',
     destination: '/platforms/javascript/tracing/span-metrics/',