@Override
  public void incrementBytes(int quantity) {
    checkScoped();
    messageBytes.set(quantity);
    bytes.increment(quantity);
  }
This is a simple metric service that exports the following to the "/metrics" endpoint:

<pre>
<ul>
    <li>counter.zipkin_collector.messages.$transport - cumulative messages received; should
relate to messages reported by instrumented apps</li>
    <li>counter.zipkin_collector.messages_dropped.$transport - cumulative messages dropped;
reasons include client disconnects or malformed content</li>
    <li>counter.zipkin_collector.bytes.$transport - cumulative message bytes</li>
    <li>counter.zipkin_collector.spans.$transport - cumulative spans read; should relate to
messages reported by instrumented apps</li>
    <li>counter.zipkin_collector.spans_dropped.$transport - cumulative spans dropped; reasons
include sampling or storage failures</li>
    <li>gauge.zipkin_collector.message_spans.$transport - last count of spans in a message</li>
    <li>gauge.zipkin_collector.message_bytes.$transport - last count of bytes in a message</li>
</ul>
</pre>

See https://docs.spring.io/spring-boot/docs/current/reference/html/production-ready-metrics.html
zipkin-server/src/main/java/zipkin2/server/internal/MicrometerCollectorMetrics.java