public ClientPreprocessorsBuilder add(HttpPreprocessor preprocessor) {
        preprocessors.add(requireNonNull(preprocessor, "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/ClientPreprocessorsBuilder.java