chore: add MPL license comments

scripts/export-metrics.js

@@ -1,3 +1,7 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
 #!/usr/bin/env node
 
 /**