[Transfer Engine] fix metadata server connection string builder in vllm intergation

[alogfans]

Mooncake supports vllm integration with etcd/redis/http as metadata service. You can change it in mooncake.json

{
  "prefill_url": "192.168.0.137:13003",
  "decode_url": "192.168.0.139:13003",
  "metadata_server": "192.168.0.139:2379", # you can add protocol prefix like "etcd://", "redis://" or "http://"
  "protocol": "rdma", # if you use protocol prefix in metadata_server, this field is ignored
  "device_name": "erdma_0"
}

[ShangmingCai]

LGTM