private static void addSelector(String clusterConfigId, String selector, DocumentProtocolPoliciesConfig.Cluster.Builder builder) {
        try {
            new DocumentSelector(selector);
        } catch (com.yahoo.document.select.parser.ParseException e) {
            throw new IllegalArgumentException("Failed to parse selector '" + selector +
                                               "' for route '" + clusterConfigId +
                                               "' in policy 'DocumentRouteSelector'.");
        }
        builder.selector(selector);
    }
Creates a service index based on the plugins loaded. This means to fill the index with all services known by this
protocol by traversing the plugins.

@param plugins All initialized plugins of the Vespa model
@return the index of all known services
config-model/src/main/java/com/yahoo/vespa/model/routing/DocumentProtocol.java