Bug 1715858 - Part2. Add telemetry scalars to count the tab unload and reload events. r=NeilDeakin

This patch adds two scalars:

* `browser.engagement.tab_unload_count`: Incremented by 1 when TabUnloader
  successfully unloaded a tab.
* `browser.engagement.tab_reload_count`: Incremented by 1 when an unloaded
  tab was reloaded.

The value of `browser.engagement.tab_reload_count` will be the same as
the count of the histogram `TAB_UNLOAD_TO_RELOAD` data, but we still need
this scalar to compare it to `browser.engagement.tab_unload_count`, which
tells us the ratio of reload-after-unload tabs to all unloaded tabs.

Depends on D120019

Differential Revision: https://phabricator.services.mozilla.com/D120020

browser/base/content/tabbrowser-tab.js

@@ -267,6 +267,7 @@
 
     updateLastUnloadedByTabUnloader() {
       this._lastUnloaded = Date.now();
+      Services.telemetry.scalarAdd("browser.engagement.tab_unload_count", 1);
     }
 
     recordTimeFromUnloadToReload() {
@@ -278,6 +279,7 @@
       Services.telemetry
         .getHistogramById("TAB_UNLOAD_TO_RELOAD")
         .add(diff_in_msec / 1000);
+      Services.telemetry.scalarAdd("browser.engagement.tab_reload_count", 1);
       delete this._lastUnloaded;
     }
 

toolkit/components/telemetry/Scalars.yaml

@@ -231,6 +231,36 @@ browser.engagement:
     record_in_processes:
       - 'main'
 
+  tab_unload_count:
+    bug_numbers:
+      - 1715858
+    description: >
+      The count of tab unload events by TabUnloader due to a low-memory situation.
+    expires: never
+    kind: uint
+    notification_emails:
+      - tkikuchi@mozilla.com
+    release_channel_collection: opt-out
+    products:
+      - 'firefox'
+    record_in_processes:
+      - 'main'
+
+  tab_reload_count:
+    bug_numbers:
+      - 1715858
+    description: >
+      The count of tab reload events by the user after unloaded.
+    expires: never
+    kind: uint
+    notification_emails:
+      - tkikuchi@mozilla.com
+    release_channel_collection: opt-out
+    products:
+      - 'firefox'
+    record_in_processes:
+      - 'main'
+
   total_uri_count:
     bug_numbers:
       - 1271313