code: public final  <T> TcpClient attr(AttributeKey<T> key, @Nullable T value) {
  		Objects.requireNonNull(key, "key");
  		return bootstrap(b -> b.attr(key, value));
  	}
docs: Inject default attribute to the future {@link Channel} connection. They will be
  available via {@link Channel#attr(AttributeKey)}.
  If the {@code value} is {@code null}, the attribute of the specified {@code key}
  is removed.
  
  @param key the attribute key
  @param value the attribute value
  @param <T> the attribute type
  
  @return a new {@link TcpClient}
  
  @see Bootstrap#attr(AttributeKey, Object)