[DPM] Get VPC size from cache service

[VanderChen]

In order to achieve automatic path switching, the VPC size needs to be obtained.
However, in the current implementation (PR #742 ), the counting issue requires accessing both caches and fetching all the VPC information. This will cause large delays when dealing with some large VPCs.
We plan to hand over this counting task to the cache service itself in the future to reduce latency.