Returns one reading of the last 30 seconds before the request. Gauge fields (currentVisitors, waitingVisitors, maxConcurrentVisitors) and state take the values of the newest metrics document in the window, and timestamp is the time of that document. Rate fields (*RatePerMinute) are the average over the window, rounded to an integer. data is null when no reading is available: no metrics document in the last 30 seconds, the newest document has no readable timestamp, or the metrics store could not be read.