asim: fix snapshot rate delay integer division

Previously, integer division was used when computing the delay for adding a
replica, resulting in shorter than expected delays. This patch improves accuracy
in tracking the delay by calculating a ratio with float. This also aligns with
how pkg/cmd/roachtest/tests/decommissionbench.go estimates decommission
duration.

(Note: This issue was just raised in Slack, not from the prototype.)

Epic: none
Release note: none

Author: wenyihu6

pkg/kv/kvserver/asim/config/settings.go

@@ -162,7 +162,8 @@ func (s *SimulationSettings) ReplicaChangeDelayFn() func(rangeSize int64, add bo
 	return func(rangeSize int64, add bool) time.Duration {
 		delay := s.ReplicaChangeBaseDelay
 		if add {
-			delay += (time.Duration(rangeSize) / time.Duration(s.RebalancingSnapshotRate)) * time.Second
+			estimatedTimeToAddReplica := float64(rangeSize) / float64(s.RebalancingSnapshotRate)
+			delay += time.Duration(estimatedTimeToAddReplica) * time.Second
 		}
 		return delay
 	}