void clear(String index) throws IOException {
    String url = '/' + index;
    AggregatedHttpRequest delete = AggregatedHttpRequest.of(HttpMethod.DELETE, url);
    http().newCall(delete, BodyConverters.NULL, "delete-index").execute();
  }
This is an internal blocking call, only used in tests.
zipkin-storage/elasticsearch/src/main/java/zipkin2/elasticsearch/ElasticsearchStorage.java