grpcproxy: Disable fast fail on lease grant call to cluster

Problem Observed
----------------

When there is no etcd process behind the proxy,
clients repeat resending lease grant requests without delay.
This behavior can cause abnormal resource consumption on CPU/RAM and
network.

Problem Detail
--------------

`LeaseGrant()` uses a bare protobuf client to forward requests.
However, it doesn't use `grpc.FailFast(false)`, which means the method returns
an `Unavailable` error immediately when no etcd process is available.
In clientv3, `Unavailable` errors are not considered the "Halt" error,
and library retries the request without delay.
Both clients and the proxy consume much CPU cycles to process retry requests.

Resolution
----------

Add `grpc.FailFast(false))` to `LeaseGrant()` of the `leaseProxy`.
This makes the proxy not to return immediately when no etcd process is
available. Clients will simply timeout requests instead.

proxy/grpcproxy/lease.go

@@ -73,7 +73,7 @@ func NewLeaseProxy(c *clientv3.Client) (pb.LeaseServer, <-chan struct{}) {
 }
 
 func (lp *leaseProxy) LeaseGrant(ctx context.Context, cr *pb.LeaseGrantRequest) (*pb.LeaseGrantResponse, error) {
-	rp, err := lp.leaseClient.LeaseGrant(ctx, cr)
+	rp, err := lp.leaseClient.LeaseGrant(ctx, cr, grpc.FailFast(false))
 	if err != nil {
 		return nil, err
 	}