Returns the cluster-time in milliseconds.
The cluster-time measures elapsed time since the cluster was created. Comparable to the System.nanoTime().
the cluster-time (elapsed milliseconds since the cluster was created).
Returns the cluster up-time in milliseconds.
When first node in cluster becomes master, its clusterTime value is saved as
clusterStartTime. Up-time of cluster is calculated as
ClusterClock#getClusterTime - clusterStartTime.