Metric Logging updates 3/N

[felipemello1]

t = Tracer()
t.start() # t0
# do_something()
t.step("t1")
# do_something()
t.stop() # t2

When measuring time with tracer, we have to capture the last time t2-t1. I was manually calling t.step("end") internally, during t.stop(), as a way to get this last step between t2-t1. This is used to compute total_time, but it should not be logged as an additional step. To drop it, i would just drop last. The logic was brittle, since when doing it cuda, we create threads that run async and dont return results in the order they are sent. This PR addresses it by dropping the "sort + drop last" logic. Instead, we create an unique name for this step, pop it from the list, and return it as a number apart.