This PR does minimal changes to interface to allow removing all
references to prometheus from `test` directory. In future I would expect
wrapping prometheus samples to provide better abstraction. Changes:
Move generic_metrics.go to testutil/metrics.go
Remove etcd.go as it was not called
Move prometheus label consts to testutil.
PrettyPrintJSON is most used e2emetrics function and that doesn't seem
specific for metrics. The implementation itself is generic, so it is
nice to move it to core framework for avoiding circular dependency.
