location-based balance doesn't work

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Set zones for tidb and tiproxy and start workload

2. What did you expect to see? (Required)

tiproxy routes to the tidb with the same zone

3. What did you see instead (Required)

tiproxy routes to each tidb with same traffic

4. What is your version? (Required)

master