@Override
    public FeedClientBuilder addProxyRequestHeader(String name, String value) {
        this.proxyRequestHeaders.put(requireNonNull(name), () -> requireNonNull(value));
        return this;
    }
Adds HTTP request header to all client requests. Value {@link Supplier} is invoked for each HTTP request,
i.e. value can be dynamically updated during a feed.
vespa-feed-client/src/main/java/ai/vespa/feed/client/impl/FeedClientBuilderImpl.java