public ClientDecorationBuilder add(DecoratingHttpClientFunction decorator) {
        requireNonNull(decorator, "decorator");
        return add(delegate -> new FunctionalDecoratingHttpClient(delegate, decorator));
    }
Adds the specified HTTP-level {@code decorator}.

@param decorator the {@link DecoratingHttpClientFunction} that intercepts an invocation
core/src/main/java/com/linecorp/armeria/client/ClientDecorationBuilder.java