public void markOperationEnd() {
    logger.trace("{} finished for {}", metrics.operationType, context);
    processingStartTime = System.currentTimeMillis();
    long operationTime = processingStartTime - operationStartTime;
    metrics.operationTimeInMs.update(operationTime);
  }
Marks that the operation being tracked has ended and callback processing has started.
ambry-utils/src/main/java/com/github/ambry/utils/AsyncOperationTracker.java