public void markRequestCompleted() {
      if (requestReceivedTime == 0) {
        throw new IllegalStateException("Request was marked completed without being marked received");
      }
      roundTripTimeInMs = System.currentTimeMillis() - requestReceivedTime;
    }
Marks the time at which request was completed so that request RTT can be calculated.
ambry-api/src/main/java/com/github/ambry/rest/RestRequestMetricsTracker.java