Add license type: Apache2 is a NOTICE license

Bug: 68860345
Bug: 69058154
Bug: 151953481

Test: no code changes

Change-Id: Iefb77e51b1b43343f7c456f7ac2868cef3b957ad
diff --git a/METADATA b/METADATA
index 9c9b51c..536369d 100644
--- a/METADATA
+++ b/METADATA
@@ -10,6 +10,7 @@
     value: "https://github.com/google/apitools"
   }
   version: "v0.5.30"
+  license_type: NOTICE
   last_upgrade_date {
     year: 2019
     month: 6