Make get_all_server_info and get_scaling_group_servers use pure-http

The metrics.py code had to be updated to deal with this, and I refactored the effectiness a little bit into metrics.py, so that there's a new get_all_metrics_effects function extracted from get_all_metrics.

I've added unit tests for get_all_metrics_effects, and left the unit tests for get_all_metrics in place (and updated them to ensure that code is using pure-HTTP properly). The tests for that are pretty nasty, but hopefully they'll go away if more of metrics.py gets Effect-ified.


I'm wondering to what extent we'd want `retry_effect` call sites to specify their own retry behavior. It seems that trying five times with exponential backoff of 2 isn't particularly specific to this function, and it may make sense to have defaults somewhere.

This clearly doesn't have to happen in this PR.