No default is applied right now, either by NewConfig nor any subsequent
code. This results in every single request being logged at warning level
as the threshold duration is zero.

Fixes: 2a26f7ae4c8 ("etcdserver: configure "expensive" requests duration")
Signed-off-by: Lorenz Brun <lorenz@monogon.tech>
(cherry picked from commit cd64a229e3ff585f6c1dc6d9cedd4efd3d24d4f8)