So far each instance of embed server was overriding the grpc loggers and zap.global loggers.
It's counter intutitive that last created Embedded server was 'wining' and more-over it was breaking grpc expectation to change it "only" before the grpc stack is being used.
This PR introduces explicit call: `embed.Config::SetupGlobalLoggers()`, that changes the loggers where requested. The call is used by etcd main binary.
The immediate benefit from this change is reduction of test flakiness, as there were flakes due to not a proper logger being used across tests.
