only track updated CLS if FCP is recorded

Author: AbhiPrasad

packages/tracing/src/browser/metrics.ts

@@ -189,6 +189,12 @@ export class MetricsInstrumentation {
         });
       }
 
+      // If FCP is not recorded we should not record the updated cls value
+      // according to the new definition of CLS.
+      if (!('fcp' in this._measurements)) {
+        delete this._measurements['updated-cls'];
+      }
+
       transaction.setMeasurements(this._measurements);
       this._tagMetricInfo(transaction);
     }
@@ -245,6 +251,9 @@ export class MetricsInstrumentation {
       this._clsEntry = entry as LayoutShift;
     });
 
+    // See:
+    // https://web.dev/evolving-cls/
+    // https://web.dev/cls-web-tooling/
     getUpdatedCLS(metric => {
       const entry = metric.entries.pop();
       if (!entry) {

packages/tracing/src/browser/web-vitals/README.md

@@ -12,6 +12,11 @@ Current vendored web vitals are:
 - FID (First Input Delay)
 - CLS (Cumulative Layout Shift)
 
+## Notable Changes from web-vitals library
+
+This vendored web-vitals library is meant to be used in conjugtion with the `@sentry/tracing` `BrowserTracing` integration.
+As such, logic around `BFCache` and multiple reports were removed from the library as our web-vitals only report once per pageload.
+
 ## License
 
 [Apache 2.0](https://github.com/GoogleChrome/web-vitals/blob/master/LICENSE)