public static Call.Mapper<List<Span>, List<List<Span>>> create(boolean strictTraceId) {
    return new GroupByTraceId(strictTraceId);
  }
A mapper that groups unorganized input spans by trace ID. Useful when preparing a result for
{@link SpanStore#getTraces(QueryRequest)}.
zipkin/src/main/java/zipkin2/storage/GroupByTraceId.java