Add Identifier message to METADATA file

Issue: 142893983
Reviewed-on: youtube/cobalt#2535

components/metrics/METADATA

@@ -3,11 +3,19 @@ description:
   "Subtree at components/metrics."
 
 third_party {
-  url {
-    type: GIT
-    value: "https://chromium.googlesource.com/chromium/src"
+  identifier {
+    type: "ChromiumVersion"
+    value: "70.0.3538.110"  # from https://chromereleases.googleblog.com/2018/11/stable-channel-update-for-desktop_19.html
+  }
+  identifier {
+    type: "Git"
+    value: "https://chromium.googlesource.com/chromium/src.git"
+    version: "ec3092c8d21e20e39698f5272c8a7f71c3ee88a2"
+  }
+  identifier {
+    type: "UpstreamSubdir"
+    value: "components/metrics"
   }
-  version: "70.0.3538.110"
   last_upgrade_date {
     year: 2018
     month: 8