Add an explicit tracking knob and expose the sampled task schedule
latency through TaskMeta. Reuse the histogram poll timestamp where
possible and preserve grouped histogram accounting for LIFO polls.
Document activation and interval semantics and cover current-thread,
multi-thread, LIFO, disabled, and non-poll callback behavior.
A consistent bit of feedback I've heard is that the `poll_count_histogram` name is a little confusing since the value customers actually get out of it is `poll_times`.
This renames all public APIs from `poll_count` to `poll_time`. The existing APIs were deprecated with one exception: the newly added `poll_count_histogram_configuration` which hasn't been released yet was simply renamed.
Russell Cohen