Adding AMP cache PageLoad.* metrics

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index bc086e693b95ff6d0e20ac3857c051ddaa80dc9b..be5e0becdfc7981874e3f71e18c8912bd87aaca5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -113009,6 +113009,21 @@ value.
       name="PageLoad.ParseTiming.NavigationToParseStart.LoadType.ForwardBackNavigation"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="PageLoadMetricsClientsAmpCachePages" separator="."
+    ordering="prefix">
+  <suffix name="Clients.AMPCache"
+      label="PageLoadMetrics that are a result of a navigations to an AMP
+             cache page. Same page navigations are not tracked."/>
+  <affected-histogram
+      name="PageLoad.DocumentTiming.NavigationToDOMContentLoadedEventFired"/>
+  <affected-histogram name="PageLoad.DocumentTiming.NavigationToFirstLayout"/>
+  <affected-histogram
+      name="PageLoad.DocumentTiming.NavigationToLoadEventFired"/>
+  <affected-histogram
+      name="PageLoad.PaintTiming.NavigationToFirstContentfulPaint"/>
+  <affected-histogram name="PageLoad.ParseTiming.NavigationToParseStart"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="PageLoadMetricsClientsCssScanner" separator="."
     ordering="prefix">
   <suffix name="Clients.CssScanner"