code: public com.google.protobuf.Any getMetadata() {
      return metadata_ == null ? com.google.protobuf.Any.getDefaultInstance() : metadata_;
    }
docs: <pre>
  Service-specific metadata associated with the operation.  It typically
  contains progress information and common metadata such as create time.
  Some services might not provide such metadata.  Any method that returns a
  long-running operation should document the metadata type, if any.
  </pre>
  
  <code>.google.protobuf.Any metadata = 2;</code>