@Override
    public void getConfig(DocumentrouteselectorpolicyConfig.Builder builder) {
        for (ContentCluster cluster : Content.getContentClusters(repo)) {
            addRoute(cluster.getConfigId(), cluster.getRoutingSelector(), builder);
        }
    }
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