Bug 1823537 - Port some older telemetry probes from the old migration wizard to the new one. r=Gijs

Those probes are:
* migration.uninstaller_profile_refresh (scalar)
* migration.discovered_migrators (keyed scalar)
* FX_MIGRATION_SOURCE_BROWSER (histogram)
* FX_MIGRATION_USAGE (histogram)
* FX_MIGRATION_ERRORS (histogram)

Not being ported are the following histograms:
* FX_STARTUP_MIGRATION_BROWSER_COUNT
* FX_STARTUP_MIGRATION_EXISTING_DEFAULT_BROWSER
* FX_STARTUP_MIGRATION_DATA_RECENCY
* FX_STARTUP_MIGRATION_USED_RECENT_BROWSER

as these are all expired.

Differential Revision: https://phabricator.services.mozilla.com/D180231

browser/components/migration/MigrationUtils.sys.mjs

@@ -618,6 +618,15 @@ class MigrationUtils {
       };
 
       if (aOptions.isStartupMigration) {
+        // Record that the uninstaller requested a profile refresh
+        if (Services.env.get("MOZ_UNINSTALLER_PROFILE_REFRESH")) {
+          Services.env.set("MOZ_UNINSTALLER_PROFILE_REFRESH", "");
+          Services.telemetry.scalarSet(
+            "migration.uninstaller_profile_refresh",
+            true
+          );
+        }
+
         openStandaloneWindow(true /* blocking */);
         return Promise.resolve();
       }