code: public CreateQueueOptions addMetadata(String key, String value) {
          this.metadata.put(key, value);
          return this;
      }
docs: Adds a key-value pair of {@link String} to the metadata collection to set
  on a queue when the queue is created. Queue metadata is a user-defined
  collection of key-value pairs that is opaque to the server. If the key
  already exists in the metadata collection, the value parameter will
  overwrite the existing value paired with that key without notification.
  <p>
  The updated metadata is only added to a newly created queue where this
  {@link CreateQueueOptions} instance is passed as a parameter.
  
  @param key
  A {@link String} containing the key part of the key-value pair
  to add to the metadata.
  @param value
  A {@link String} containing the value part of the key-value
  pair to add to the metadata.
  @return A reference to this {@link CreateQueueOptions} instance.