List<Span> sample(List<Span> input) {
    List<Span> sampled = new ArrayList<>(input.size());
    for (int i = 0, length = input.size(); i < length; i++) {
      Span s = input.get(i);
      if (sampler.isSampled(s.traceId(), Boolean.TRUE.equals(s.debug()))) {
        sampled.add(s);
      }
    }
    int dropped = input.size() - sampled.size();
    if (dropped > 0) metrics.incrementSpansDropped(dropped);
    return sampled;
  }
Before calling this, call {@link CollectorMetrics#incrementMessages()}, and {@link
CollectorMetrics#incrementBytes(int)}. Do not call any other metrics callbacks as those are
handled internal to this method.

@param serializedSpans not empty message
zipkin-collector/core/src/main/java/zipkin2/collector/Collector.java