public void markFirstByteSent() {
      if (requestReceivedTime == 0) {
        throw new IllegalStateException("First response byte was marked as sent without request being marked received");
      }
      timeToFirstByteInMs = System.currentTimeMillis() - requestReceivedTime;
    }
Marks the time at which the first byte of the response is sent.
ambry-api/src/main/java/com/github/ambry/rest/RestRequestMetricsTracker.java