tracing-subscriber: count numbers of enters in Timings

[mladedav]

Motivation

The timings measurements were done naively in a way where they added busy time in on_exit and idle time in `on_enter. This works only if the spans are not entered multiple times. For example if a span is:

entered
wait 1 second
entered
exited
exited

The timing would be about 1 second of idle time and no busy time.

Solution

Count the relative number of enters and exits and only increment the timers on top-level enters and exits.

[hawkw]

yup, this is what we do in tokio-console, so it makes sense to do it here as well! thanks!