@Override
    public <V> RequestOptionsBuilder attr(AttributeKey<V> key, @Nullable V value) {
        requireNonNull(key, "key");

        if (attributes == null) {
            attributes = new HashMap<>();
        }

        if (value == null) {
            attributes.remove(key);
        } else {
            attributes.put(key, value);
        }
        return this;
    }
A builder for creating a new {@link RequestOptions}.
core/src/main/java/com/linecorp/armeria/client/RequestOptionsBuilder.java