@UnstableApi
    public AbstractClientOptionsBuilder rpcPreprocessor(RpcPreprocessor rpcPreprocessor) {
        clientPreprocessorsBuilder.addRpc(rpcPreprocessor);
        return this;
    }
Adds the specified RPC-level {@code rpcPreprocessor}.

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