private static void addIndexingHop(List<ContentCluster> content, RoutingTableSpec table) {
        if (content.isEmpty()) return;

        HopSpec hop = new HopSpec("indexing", "[DocumentRouteSelector]");
        for (ContentCluster cluster : content) {
            hop.addRecipient(cluster.getConfigId());
        }
        if (hop.hasRecipients()) {
            table.addHop(hop);
        }
    }
Create the "indexing" hop. This hop contains all non-streaming search clusters as recipients, and the routing
policy "SearchCluster" will decide which cluster(s) are to receive every document passed through it based on a
document select string derived from services.xml.

@param table the routing table to add to
config-model/src/main/java/com/yahoo/vespa/model/routing/DocumentProtocol.java