@Override
  public String get(TracingProtos.RPCTInfo carrier, String key) {
    return Optional.ofNullable(carrier).map(TracingProtos.RPCTInfo::getHeadersMap)
      .map(map -> map.get(key)).orElse(null);
  }
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