CassandraSpanConsumer spanConsumer(CassandraStorage.Builder builder) {
    return new CassandraSpanConsumer(session, metadata, builder.strictTraceId,
      builder.searchEnabled, builder.autocompleteKeys, builder.autocompleteTtl,
      builder.autocompleteCardinality);
  }
To allow lookups w/o a span name, we index "". "" is used instead of null to avoid creating
tombstones.
zipkin-storage/cassandra/src/test/java/zipkin2/storage/cassandra/CassandraSpanConsumerTest.java