public void getConfig(IndexInfoConfig.Builder builder) {
            for (DocumentDatabase docDb : docDbs) {
                docDb.getConfig(builder);
            }
        }
Class used to retrieve combined configuration from multiple document databases.
It is not a direct {@link ConfigInstance.Producer} of those configs,
that is handled (by delegating to this) by the {@link IndexedSearchCluster}
which is the parent to this. This avoids building the config multiple times.
config-model/src/main/java/com/yahoo/vespa/model/search/SearchCluster.java