feat(core): Add beforeSendSpan documentation for javascript (#10055)

docs/platforms/javascript/common/configuration/filtering.mdx

@@ -111,3 +111,9 @@ Learn more about <PlatformLink to="/configuration/sampling/">configuring the sam
 ### Using <PlatformIdentifier name="before-send-transaction" />
 
 <PlatformContent includePath="configuration/before-send-transaction" />
+
+## Filtering Spans
+
+To prevent certain spans from being reported to Sentry, use the <PlatformIdentifier name="before-send-span" /> configuration option, which allows you to provide a function to evaluate the current span and drop it if it's not one you want.
+
+<PlatformContent includePath="configuration/before-send-span" />

docs/platforms/javascript/common/configuration/options.mdx

@@ -300,6 +300,12 @@ The callback typically gets a second argument (called a "hint") which contains t
 
 </ConfigKey>
 
+<ConfigKey name="before-send-span">
+
+This function is called with an SDK-specific span object, and can return a modified span object, or `null` to skip reporting the span. One way this might be used is for manual PII stripping before sending.
+
+</ConfigKey>
+
 
 ## Transport Options
 

platform-includes/configuration/before-send-span/javascript.mdx

@@ -0,0 +1,17 @@
+<SignInNote />
+
+```javascript
+Sentry.init({
+  dsn: "___PUBLIC_DSN___",
+
+  // Called for spans
+  beforeSendSpan(span) {
+    // Modify or drop the span here
+    if (span.description === 'unimportant span') {
+      // Don't send the span to Sentry
+      return null;
+    }
+    return span;
+  },
+});
+```