static long guessTimestamp(Span span) {
    assert 0L == span.timestampAsLong() : "method only for when span has no timestamp";
    for (Annotation annotation : span.annotations()) {
      if (0L < annotation.timestamp()) return annotation.timestamp();
    }
    return 0L; // return a timestamp that won't match a query
  }
This fans out into many requests, last count was 2 * spans.size. If any of these fail, the
returned future will fail. Most callers drop or log the result.
zipkin-storage/cassandra/src/main/java/zipkin2/storage/cassandra/CassandraSpanConsumer.java