For #22155 - Inactive tabs count telemetry

A quantity probe in the metrics ping means we'll loose the granularity events
provided but it will be easier to extract the values.

For reporting whether the inactive tabs feature is enabled or not we already
have the "preferences.inactive_tabs_enabled" probe so I didn't duplicate this.

app/metrics.yaml

@@ -1315,6 +1315,23 @@ metrics:
     notification_emails:
       - android-probes@mozilla.com
     expires: "2022-02-01"
+  inactive_tabs_count:
+    type: quantity
+    lifetime: application
+    description: |
+      How many inactive tabs does the user have.
+    send_in_pings:
+      - metrics
+    bugs:
+      - https://github.com/mozilla-mobile/fenix/issues/22155
+    data_reviews:
+      - https://github.com/mozilla-mobile/fenix/pull/22163#issuecomment-957636802
+    data_sensitivity:
+      - interaction
+    notification_emails:
+      - android-probes@mozilla.com
+    expires: "2022-02-01"
+    unit: integer
 
 customize_home:
   most_visited_sites:

app/src/main/java/org/mozilla/fenix/FenixApplication.kt

@@ -81,6 +81,7 @@ import org.mozilla.fenix.components.metrics.Event
 import org.mozilla.fenix.components.metrics.MozillaProductDetector
 import org.mozilla.fenix.components.toolbar.ToolbarPosition
 import org.mozilla.fenix.perf.MarkersActivityLifecycleCallbacks
+import org.mozilla.fenix.tabstray.ext.inactiveTabs
 import org.mozilla.fenix.utils.Settings
 
 /**
@@ -627,6 +628,8 @@ open class FenixApplication : LocaleAwareApplication(), Provider {
             tabViewSetting.set(settings.getTabViewPingString())
             closeTabSetting.set(settings.getTabTimeoutPingString())
 
+            inactiveTabsCount.set(browserStore.state.inactiveTabs.size.toLong())
+
             val installSourcePackage = if (SDK_INT >= Build.VERSION_CODES.R) {
                 packageManager.getInstallSourceInfo(packageName).installingPackageName
             } else {

app/src/test/java/org/mozilla/fenix/FenixApplicationTest.kt

@@ -11,6 +11,7 @@ import io.mockk.Runs
 import io.mockk.every
 import io.mockk.just
 import io.mockk.mockk
+import io.mockk.mockkStatic
 import io.mockk.spyk
 import io.mockk.verify
 import mozilla.components.browser.state.store.BrowserStore
@@ -32,6 +33,7 @@ import org.mozilla.fenix.GleanMetrics.SearchDefaultEngine
 import org.mozilla.fenix.components.metrics.MozillaProductDetector
 import org.mozilla.fenix.components.toolbar.ToolbarPosition
 import org.mozilla.fenix.helpers.FenixRobolectricTestRunner
+import org.mozilla.fenix.tabstray.ext.inactiveTabs
 import org.mozilla.fenix.utils.BrowsersCache
 import org.mozilla.fenix.utils.Settings
 import org.robolectric.annotation.Config
@@ -141,8 +143,13 @@ class FenixApplicationTest {
         every { settings.searchTermTabGroupsAreEnabled } returns true
         every { application.reportHomeScreenMetrics(settings) } just Runs
         every { settings.inactiveTabsAreEnabled } returns true
+        mockkStatic("org.mozilla.fenix.tabstray.ext.TabSelectorsKt") {
+            every { browserStore.state.inactiveTabs } returns listOf(mockk(), mockk())
 
-        application.setStartupMetrics(browserStore, settings, browsersCache, mozillaProductDetector)
+            application.setStartupMetrics(browserStore, settings, browsersCache, mozillaProductDetector)
+
+            assertEquals(2, Metrics.inactiveTabsCount.testGetValue())
+        }
 
         // Verify that browser defaults metrics are set.
         assertEquals("Mozilla", Metrics.distributionId.testGetValue())