discard empty async task spans

[FrankChen021]

Some scheduled tasks may fork too many async too many tasks. but these async tasks don't contain child span logs.
It looks like meaningless to collect these logs.

Impact

1. may take too much memory
2. may use out the collection queue that causing discard of span logs in queue
3. takes more space at server side to store them

Solution:
Maybe we can discard these empty tasks, and hold a counter at the parent span which forks sub tasks to indicate how many sub tasks have been forked.