@Override
  public Iterable<String> keys(TracingProtos.RPCTInfo carrier) {
    return Optional.ofNullable(carrier).map(TracingProtos.RPCTInfo::getHeadersMap).map(Map::keySet)
      .orElse(Collections.emptySet());
  }
Used to extract a tracing {@link Context} from an instance of {@link TracingProtos.RPCTInfo}.
hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/RPCTInfoGetter.java