This log allows us to easily distinguish between full coverage and
minority partition when the cluster fails. Sometimes it is not easy
to see the minority partition in a healthy shards (both primary and
replicas).
And we decided not to add a cluster_fail_reason field to cluster info.
Given that there are only two reasons and both are well-known and if
we ended up adding more down the road we can add it in the furture.
Signed-off-by: Binbin <binloveplay1314@qq.com>