[server] Do not log fast health check calls.
[server] Do not log fast health check calls.

They mostly spam logs. Still log slow calls though, since they likely indicate
a problem.

R=a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org
BUG=959427

Change-Id: I59a9c5bfafccc891025f6d4a962acfbc36800a8a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1672530
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go