Documentation: Tweak etcdMembersDown to reduce false negatives
Documentation: Tweak etcdMembersDown to reduce false negatives

Before this change, during a reboot in which etcd recovers quickly (e.g. 1 min),
the etcdMembersDown alert tends to fire even when etcd is fully healthy because
the averaging function can take more than 3 minutes to average back down below
the 0.01 threshold.

This change tries to reduce the possibility of a false negative by considering a
shorter (1 min) failure rate window which tends to average down below the
threshold far more quickly (within 1 min). The `for` clause of the alert should
ensure that the alert still fires if the poor conditions are sustained for an
unreasonable overall time (3 min).

coreos/etcd,mkumatag/etcd,mqliang/etcd,dannysauer/etcd,gyuho/etcd,mkumatag/etcd,dannysauer/etcd,etcd-io/etcd,rnaveiras/etcd,mqliang/etcd,coreos/etcd,li-ang/etcd,ajityagaty/etcd,ajityagaty/etcd,etcd-io/etcd,etcd-io/etcd,rnaveiras/etcd,li-ang/etcd,mkumatag/etcd,mqliang/etcd,gyuho/etcd,ajityagaty/etcd,rnaveiras/etcd