Emit counter events

[kayceesrk]

OCaml runtime emits counter events already. This PR emits them to the trace.

[kayceesrk]

(With some additional events that the OCaml runtime doesn't emit today,) one can clearly see counter events and the spans telling a nice story

In the above, the major slice is triggered when there is a spike in allocation (allocated_words, allocation_counter). Since the work done by GC (work_counter) lags behind, a slice gets triggered after which the work_counter catches up.

[tmcgilchrist]

That looks really useful. Thanks for adding it. 👍🏻