@Override
    public FeedClientBuilderImpl addRequestHeader(String name, Supplier<String> valueSupplier) {
        this.requestHeaders.put(requireNonNull(name), requireNonNull(valueSupplier));
        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