fix(browser): Skip redundant CLS/LCP handlers when span streaming is enabled

When span streaming handles CLS/LCP, `startTrackingWebVitals` no longer
registers throwaway `_trackCLS()`/`_trackLCP()` handlers. Instead of
adding a separate skip flag, the existing `recordClsStandaloneSpans` and
`recordLcpStandaloneSpans` options now accept `undefined` to mean "skip
entirely" — three states via two flags instead of three flags.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: logaretm

packages/browser-utils/src/metrics/browserMetrics.ts

@@ -75,8 +75,18 @@ let _lcpEntry: LargestContentfulPaint | undefined;
 let _clsEntry: LayoutShift | undefined;
 
 interface StartTrackingWebVitalsOptions {
-  recordClsStandaloneSpans: boolean;
-  recordLcpStandaloneSpans: boolean;
+  /**
+   * When `true`, CLS is tracked as a standalone span. When `false`, CLS is
+   * recorded as a measurement on the pageload span. When `undefined`, CLS
+   * tracking is skipped entirely (e.g. because span streaming handles it).
+   */
+  recordClsStandaloneSpans: boolean | undefined;
+  /**
+   * When `true`, LCP is tracked as a standalone span. When `false`, LCP is
+   * recorded as a measurement on the pageload span. When `undefined`, LCP
+   * tracking is skipped entirely (e.g. because span streaming handles it).
+   */
+  recordLcpStandaloneSpans: boolean | undefined;
   client: Client;
 }
 
@@ -97,9 +107,22 @@ export function startTrackingWebVitals({
     if (performance.mark) {
       WINDOW.performance.mark('sentry-tracing-init');
     }
-    const lcpCleanupCallback = recordLcpStandaloneSpans ? trackLcpAsStandaloneSpan(client) : _trackLCP();
+
+    const lcpCleanupCallback =
+      recordLcpStandaloneSpans === true
+        ? trackLcpAsStandaloneSpan(client)
+        : recordLcpStandaloneSpans === false
+          ? _trackLCP()
+          : undefined;
+
+    const clsCleanupCallback =
+      recordClsStandaloneSpans === true
+        ? trackClsAsStandaloneSpan(client)
+        : recordClsStandaloneSpans === false
+          ? _trackCLS()
+          : undefined;
+
     const ttfbCleanupCallback = _trackTtfb();
-    const clsCleanupCallback = recordClsStandaloneSpans ? trackClsAsStandaloneSpan(client) : _trackCLS();
 
     return (): void => {
       lcpCleanupCallback?.();

packages/browser/src/tracing/browserTracingIntegration.ts

@@ -494,8 +494,8 @@ export const browserTracingIntegration = ((options: Partial<BrowserTracingOption
       const spanStreamingEnabled = hasSpanStreamingEnabled(client);
 
       _collectWebVitals = startTrackingWebVitals({
-        recordClsStandaloneSpans: !spanStreamingEnabled && (enableStandaloneClsSpans || false),
-        recordLcpStandaloneSpans: !spanStreamingEnabled && (enableStandaloneLcpSpans || false),
+        recordClsStandaloneSpans: spanStreamingEnabled ? undefined : enableStandaloneClsSpans || false,
+        recordLcpStandaloneSpans: spanStreamingEnabled ? undefined : enableStandaloneLcpSpans || false,
         client,
       });