@PipelineProperty(
            description = "Name of the Elasticsearch index. Variables specified such as `{year}` are replaced with " +
                          "the corresponding field values contained in the document root. Field names beginning with " +
                          "an underscore are not written to the document and are only used in the index name pattern.",
            displayPriority = 5
    )
    public void setIndexName(final String indexName) {
        this.indexName = indexName;
    }
Build the index name, substituting values from the source document where specified.
For instance, consider an index name of `stroom-index-{_year}`. A field `<string key='_year'>2023</string>` in
the doc root will result in a destination index of `stroom-index-2023` for that document. In this case, because
the field `_year` starts with an underscore, it is not rendered to the doc and instead is solely used in the
index name.
stroom-search/stroom-search-elastic/src/main/java/stroom/search/elastic/indexing/ElasticIndexingFilter.java