auth: Keep audit log of all generated delegation tokens.

appengine/components/components/auth/proto/delegation.proto

Index: appengine/components/components/auth/proto/delegation.proto
diff --git a/appengine/components/components/auth/proto/delegation.proto b/appengine/components/components/auth/proto/delegation.proto
index 6521d6b91a24d6aa10382c9121205e7a88715568..9bf8dff5297d3d7f9312f7fca275bcff03ac6d91 100644
--- a/appengine/components/components/auth/proto/delegation.proto
+++ b/appengine/components/components/auth/proto/delegation.proto
@@ -68,9 +68,10 @@ message Subtoken {
   // How long the token is considered valid (in seconds). Required.
   optional int32 validity_duration = 3;
 
-  // The token is no longer valid if at least one of these IDs is revoked.
-  // TODO(vadimsh): Implement.
-  // repeated uint64 revocation_id = 4;
+  // Identifier of this subtoken.
+  //
+  // Used for logging and tracking purposes.
+  optional int64 subtoken_id = 4;
 
   // To WHOM the authority is delegated. Each item can be an identity string
   // (e.g. "user:<email>") or a "group:<name>" string. "Any bearer can use