@UnstableApi
    @Nullable
    public <T> T attr(AttributeKey<T> key) {
        requireNonNull(key, "key");
        if (attributes == null) {
            return null;
        }
        return attributes.attr(key);
    }
Returns the attribute value associated with the given {@link AttributeKey} of this endpoint, or
{@code null} if there's no value associated with this key.
core/src/main/java/com/linecorp/armeria/client/Endpoint.java