code: def put_metadata(self, key, value, namespace='default'):
          
          entity = self.get_trace_entity()
          if entity and entity.sampled:
              entity.put_metadata(key, value, namespace)
docs: Add metadata to the current active trace entity.
          Metadata is not indexed but can be later retrieved
          by BatchGetTraces API.
  
          :param str namespace: optional. Default namespace is `default`.
              It must be a string and prefix `AWS.` is reserved.
          :param str key: metadata key under specified namespace
          :param object value: any object that can be serialized into JSON string