Bug 1647876 - Record how long users interact with Firefox Desktop r=janerik

Differential Revision: https://phabricator.services.mozilla.com/D81188

toolkit/components/telemetry/Histograms.json

@@ -15373,5 +15373,18 @@
     "bug_numbers": [1627708],
     "description": "Input device that triggered the zoom gesture (InputData::PinchGestureSource)",
     "releaseChannelCollection": "opt-out"
+  },
+  "FOG_EVAL_USER_ACTIVE_S" : {
+    "products": ["firefox"],
+    "record_in_processes": ["main"],
+    "alert_emails": ["chutten@mozilla.com"],
+    "expires_in_version": "85",
+    "kind": "exponential",
+    "keyed": false,
+    "high": 7200,
+    "n_buckets": 50,
+    "bug_numbers": [1647876],
+    "description": "Seconds between `user-interaction-active` and `user-interaction-inactive`. For use in evaluating baseline engagement for Project FOG.",
+    "releaseChannelCollection": "opt-out"
   }
 }

toolkit/components/telemetry/Scalars.yaml

@@ -4985,6 +4985,40 @@ migration:
     operating_systems:
       - windows
 
+fog.eval:
+  user_active:
+    bug_numbers:
+      - 1647876
+    description: >
+      Whether the user is presently actively interacting with Firefox.
+      For use in evaluating baseline engagement for Project FOG.
+    expires: '85'
+    kind: 'boolean'
+    notification_emails:
+      - 'chutten@mozilla.com'
+    release_channel_collection: 'opt-out'
+    products:
+      - 'firefox'
+    record_in_processes:
+      - 'main'
+
+  user_active_error:
+    bug_numbers:
+      - 1647876
+    description: >
+      How many Telemetry stopwatch errors did we have while trying to measure user activity?
+      These could be attempts to start a started timer or finish an unstarted timer or so forth.
+      See Stopwatch::Start and Stopwatch::Finish for details.
+      For use in evaluating baseline engagement for Project FOG.
+    expires: '85'
+    kind: 'uint'
+    notification_emails:
+      - 'chutten@mozilla.com'
+    release_channel_collection: 'opt-out'
+    products:
+      - 'firefox'
+    record_in_processes:
+      - 'main'
 
 # The following section is for probes testing the Telemetry system. They will not be
 # submitted in pings and are only used for testing.

toolkit/components/telemetry/pings/TelemetrySession.jsm

@@ -321,6 +321,10 @@ var Impl = {
   // The activity state for the user. If false, don't count the next
   // active tick. Otherwise, increment the active ticks as usual.
   _isUserActive: true,
+  // The activity state for the user. Inits to false so that the initial
+  // activity doesn't cause a stopwatch error.
+  // Used to evaluate FOG user engagement.
+  _fogUserActive: false,
   _startupIO: {},
   // The previous build ID, if this is the first run with a new build.
   // Null if this is the first run, or the previous build ID is unknown.
@@ -1147,6 +1151,10 @@ var Impl = {
    */
   _onActiveTick(aUserActive) {
     const needsUpdate = aUserActive && this._isUserActive;
+    const userActivityChanged =
+      (aUserActive && !this._fogUserActive) ||
+      (!aUserActive && this._fogUserActive);
+    this._fogUserActive = aUserActive;
     this._isUserActive = aUserActive;
 
     // Don't count the first active tick after we get out of
@@ -1155,6 +1163,22 @@ var Impl = {
       this._sessionActiveTicks++;
       Telemetry.scalarAdd("browser.engagement.active_ticks", 1);
     }
+
+    if (userActivityChanged) {
+      // FOG User Engagement Evaluation.
+      Telemetry.scalarSet("fog.eval.user_active", aUserActive);
+      let error = false;
+      if (aUserActive) {
+        error = !TelemetryStopwatch.start("FOG_EVAL_USER_ACTIVE_S", null, {
+          inSeconds: true,
+        });
+      } else {
+        error = !TelemetryStopwatch.finish("FOG_EVAL_USER_ACTIVE_S");
+      }
+      if (error) {
+        Telemetry.scalarAdd("fog.eval.user_active_error", 1);
+      }
+    }
   },
 
   /**