perfetto: allow building metrics protos in Java using Android

This unblocks work integrating Perfetto into tradefed.

Change-Id: I32de25b4296df8b186cd553bdba8289b8303dc10
diff --git a/Android.bp.extras b/Android.bp.extras
index ec61a44..9586516 100644
--- a/Android.bp.extras
+++ b/Android.bp.extras
@@ -29,6 +29,16 @@
   ],
 }
 
+java_library_host {
+  name: "perfetto_metrics-full",
+  proto: {
+      type: "full",
+  },
+  srcs: [
+    "protos/perfetto/metrics/perfetto_merged_metrics.proto",
+  ],
+}
+
 // This sample target shows how to use the perfetto client API from within the
 // Android tree.
 cc_binary {