@UnstableApi
    public AbstractClientOptionsBuilder preprocessor(HttpPreprocessor preprocessor) {
        clientPreprocessorsBuilder.add(preprocessor);
        return this;
    }
Adds the specified HTTP-level {@code preprocessor}.

@param preprocessor the {@link HttpPreprocessor} that preprocesses an invocation
core/src/main/java/com/linecorp/armeria/client/AbstractClientOptionsBuilder.java