A Probe records some integer value that is passed to at along with a
timestamp.
type 'a create_argsThe *_args below are instantiated differently for Timers and Probes. See
Profiler_intf below.
module Group : sig ... endA Group provides a way of grouping multiple probes (or timers). Once grouped, one
can measure stats between members of a group -- i.e. the time it takes to get from
one probe to the other in the group, or the change in a metric between two probes in
a group.