Bug 1862939 - Part 2: Add LCP information to the Pageload Event. r=dpalmeiro

Differential Revision: https://phabricator.services.mozilla.com/D192695

dom/base/Document.cpp

@@ -2174,6 +2174,14 @@ void Document::AccumulatePageLoadTelemetry(
     }
   }
 
+  // Report the most up to date LCP time. For our histogram we actually report
+  // this on page unload.
+  if (TimeStamp lcpTime =
+          GetNavigationTiming()->GetLargestContentfulRenderTimeStamp()) {
+    aEventTelemetryDataOut.lcpTime = mozilla::Some(
+        static_cast<uint32_t>((lcpTime - navigationStart).ToMilliseconds()));
+  }
+
   // DOM Content Loaded event
   if (TimeStamp dclEventStart =
           GetNavigationTiming()->GetDOMContentLoadedEventStartTimeStamp()) {

dom/ipc/PageLoadEventUtils.h

@@ -18,6 +18,7 @@ struct ParamTraits<mozilla::glean::perf::PageLoadExtra> {
 
   static void Write(MessageWriter* aWriter, const paramType& aParam) {
     WriteParam(aWriter, aParam.fcpTime);
+    WriteParam(aWriter, aParam.lcpTime);
     WriteParam(aWriter, aParam.jsExecTime);
     WriteParam(aWriter, aParam.loadTime);
     WriteParam(aWriter, aParam.loadType);
@@ -32,6 +33,7 @@ struct ParamTraits<mozilla::glean::perf::PageLoadExtra> {
 
   static bool Read(MessageReader* aReader, paramType* aResult) {
     return ReadParam(aReader, &aResult->fcpTime) &&
+           ReadParam(aReader, &aResult->lcpTime) &&
            ReadParam(aReader, &aResult->jsExecTime) &&
            ReadParam(aReader, &aResult->loadTime) &&
            ReadParam(aReader, &aResult->loadType) &&

dom/metrics.yaml

@@ -19,10 +19,12 @@ perf:
       - https://bugzilla.mozilla.org/show_bug.cgi?id=1759744
       - https://bugzilla.mozilla.org/show_bug.cgi?id=1799727
       - https://bugzilla.mozilla.org/show_bug.cgi?id=1834774
+      - https://bugzilla.mozilla.org/show_bug.cgi?id=1862939
     data_reviews:
       - https://bugzilla.mozilla.org/show_bug.cgi?id=1759744#c5
       - https://bugzilla.mozilla.org/show_bug.cgi?id=1799727#c4
       - https://bugzilla.mozilla.org/show_bug.cgi?id=1834774#c3
+      - https://bugzilla.mozilla.org/show_bug.cgi?id=1862939#c5
     notification_emails:
       - perf-telemetry-alerts@mozilla.com
       - dpalmeiro@mozilla.com
@@ -48,6 +50,11 @@ perf:
           "Time between firstContentfulPaint and navigationStart, in ms."
         type: quantity
         unit: ms
+      lcp_time:
+        description:
+          "Time between largestContentfulPaint and navigationStart, at the point of onLoad firing, in ms. This may differ from the final LCP value as reported through the LCP histogram."
+        type: quantity
+        unit: ms
       js_exec_time:
         description:
           "Time spent executing JS during page load, in ms."