docs: private double phi(long timestampMillis) {
          long timeDiffMillis;
          double meanMillis;
          double stdDeviationMillis;
  
          synchronized (heartbeatHistory) {
              long lastTimestampMillis = lastHeartbeatMillis;
              if (lastTimestampMillis == NO_HEARTBEAT_TIMESTAMP) {
                  return 0.0;
              }
  
              timeDiffMillis = timestampMillis - lastTimestampMillis;
              meanMillis = heartbeatHistory.mean();
              stdDeviationMillis = ensureValidStdDeviation(heartbeatHistory.stdDeviation());
          }
  
          return phi(timeDiffMillis, meanMillis + acceptableHeartbeatPauseMillis, stdDeviationMillis);
      }