private static void createDocprocChainSpec(ApplicationSpec spec,
                                               List<DocprocChain> docprocChains,
                                               List<? extends Container> containerNodes) {
        for (DocprocChain chain: docprocChains) {
            for (Container node: containerNodes)
                spec.addService(NAME, node.getConfigId() + "/chain." + chain.getComponentId().stringValue());
        }
    }
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