code: @SuppressWarnings("unchecked")
      public <T> Http2StreamChannelBootstrap attr(AttributeKey<T> key, T value) {
          if (key == null) {
              throw new NullPointerException("key");
          }
          if (value == null) {
              synchronized (attrs) {
                  attrs.remove(key);
              }
          } else {
              synchronized (attrs) {
                  attrs.put(key, value);
              }
          }
          return this;
      }
docs: Allow to specify an initial attribute of the newly created {@link Http2StreamChannel}.  If the {@code value} is
  {@code null}, the attribute of the specified {@code key} is removed.