public EsIndex build() {
            requireNonNull(indexName, "Index name must not be null");
            checkArgument(rootEntryMap.size() > 0, "Must have some mapping entries");
            return new EsIndex(this);
        }
Build and validate instance of {@link EsIndex}
@return a new EsIndex with the configuration provided in this builder
common/es/src/main/java/io/apiman/common/es/util/builder/index/EsIndex.java