private String clientVersion() {
            return Optional.ofNullable(request.headers().get(Headers.CLIENT_VERSION))
                    .filter(l -> !l.isEmpty()).map(l -> l.get(0))
                    .orElse("unknown");
        }
Returns the last property with the given name, if present, or throws if this is empty or blank.
vespaclient-container-plugin/src/main/java/com/yahoo/document/restapi/resource/DocumentV1ApiHandler.java