chore: adds throttled to UpdateDatabaseDdlMetadata

Indicates whether the ddl operation was throttled, due to resource constraints. When resources become available the operation will resume and this field will be false again.

PiperOrigin-RevId: 344883170

google/spanner/admin/database/v1/spanner_database_admin.proto

@@ -514,6 +514,11 @@ message UpdateDatabaseDdlMetadata {
   // succeeded so far, where `commit_timestamps[i]` is the commit
   // timestamp for the statement `statements[i]`.
   repeated google.protobuf.Timestamp commit_timestamps = 3;
+
+  // Output only. When true, indicates that the operation is throttled e.g
+  // due to resource constraints. When resources become available the operation
+  // will resume and this field will be false again.
+  bool throttled = 4 [(google.api.field_behavior) = OUTPUT_ONLY];
 }
 
 // The request for [DropDatabase][google.spanner.admin.database.v1.DatabaseAdmin.DropDatabase].