force report lcp on finish

Author: AbhiPrasad

packages/tracing/src/browser/metrics.ts

@@ -21,6 +21,7 @@ export class MetricsInstrumentation {
   private _performanceCursor: number = 0;
   private _lcpEntry: LargestContentfulPaint | undefined;
   private _clsEntry: LayoutShift | undefined;
+  private _reportLCP: ReturnType<typeof getLCP>;
 
   public constructor(private _reportAllChanges: boolean = false) {
     if (!isNodeEnv() && global && global.performance && global.document) {
@@ -43,6 +44,12 @@ export class MetricsInstrumentation {
 
     logger.log('[Tracing] Adding & adjusting spans using Performance API');
 
+    // `addPerformanceEntries` should be called on transaction.finish
+    // If we are finishing a transaction, we should force report the LCP.
+    if (this._reportLCP) {
+      this._reportLCP(true);
+    }
+
     const timeOrigin = msToSec(browserPerformanceTimeOrigin);
 
     let responseStartTimestamp: number | undefined;
@@ -223,7 +230,7 @@ export class MetricsInstrumentation {
 
   /** Starts tracking the Largest Contentful Paint on the current page. */
   private _trackLCP(): void {
-    getLCP(metric => {
+    this._reportLCP = getLCP(metric => {
       const entry = metric.entries.pop();
       if (!entry) {
         return;

packages/tracing/src/browser/web-vitals/getLCP.ts

@@ -34,10 +34,13 @@ export interface LargestContentfulPaint extends PerformanceEntry {
 
 const reportedMetricIDs: Record<string, boolean> = {};
 
-export const getLCP = (onReport: ReportHandler, reportAllChanges?: boolean): void => {
+export const getLCP = (
+  onReport: ReportHandler,
+  reportAllChanges?: boolean,
+): ReturnType<typeof bindReporter> | undefined => {
   const visibilityWatcher = getVisibilityWatcher();
   const metric = initMetric('LCP');
-  let report: ReturnType<typeof bindReporter>;
+  let report: ReturnType<typeof bindReporter> | undefined;
 
   const entryHandler = (entry: PerformanceEntry): void => {
     // The startTime attribute returns the value of the renderTime if it is not 0,
@@ -66,7 +69,9 @@ export const getLCP = (onReport: ReportHandler, reportAllChanges?: boolean): voi
         po.takeRecords().map(entryHandler as PerformanceEntryHandler);
         po.disconnect();
         reportedMetricIDs[metric.id] = true;
-        report(true);
+        if (report) {
+          report(true);
+        }
       }
     };
 
@@ -79,4 +84,6 @@ export const getLCP = (onReport: ReportHandler, reportAllChanges?: boolean): voi
 
     onHidden(stopListening, true);
   }
+
+  return report;
 };