labels.zone set to tiproxy instance fail when balance policy= 'location'

[seiya-annie]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create premium cluster with tiproxy config:
[balance]
label-name = "keyspace"
policy = "location"

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

check tiproxy labels in logs, zone label and value should included in the [labels] structure.

3. What did you see instead (Required)

no "zone" in the [labels] structure.

4. What is your version? (Required)

premium cluster
version: v26.3.99
tag: v26.3.1