title: mongoDB get execution time of a query in microseconds
body: <p>I should get the execution time of a query in mongoDB. Using explain() I noticed that it is zero millis. So, Is it possible in mongoDB to obtain the execution time of a query in microseconds in a simple way?
  For example if I wanted to calculate mean and variance of a sample of 10k test queries (where each query lasts micros), how could I do?
  I'm using PHP.</p>