Better metrics about sleep and group

[mstoykov]

Currently if a group includes a 10 second sleep we are going to say that the group took 10 seconds, which while correct might not be what the enduser is interested in.
I propose that we:

1. implement New metric to measure sleep times #880
2. Have the group know that there was n seconds of sleep in it's current execution - probably through some new methods on lib.Group
3. Add new metric group_sleep?
4. Choose whether to remove the sleep time from the group_duration or add another metric that is the group_duration without the group_sleep or the reverse - have the group_duration be without the group_sleep but have another full_group_duration that includes the group_sleep

[markjmeier]

In my past experience working with users of the LI v3.0 product, I never really encountered anyone who was interested in knowing sleep times within their script. Quite frankly, I don't think it's that interesting to keep track of - it's just measuring idle time. For users that do desire to know it, it shouldn't be that difficult to create a custom metric and calculate how long the sleep times are within any given script.

If there are cases where it is interesting, I'd love to know them. But I think just making sure groups deduct sleep time from their total time would improve the usefulness of group time.