config, balance: add config for labels

[djshow832]

What problem does this PR solve?

Issue Number: close #535

Problem Summary:
To implement label-based balance, we must add labels for TiProxy.

What is changed and how it works:

• Add labels in config
• Pass the config from namespace to router to balancePolicy to factors

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

- Add `labels` in configurations.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 56.25000% with 7 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@4fb56d7). Click here to learn what that means.

Files	Patch %	Lines
pkg/balance/factor/factor_balance.go	66.66%	2 Missing ⚠️
pkg/balance/policy/simple_policy.go	0.00%	2 Missing ⚠️
pkg/balance/factor/factor_conn.go	0.00%	1 Missing ⚠️
pkg/balance/factor/factor_cpu.go	0.00%	1 Missing ⚠️
pkg/balance/factor/factor_health.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #536   +/-   ##
=======================================
  Coverage        ?   65.71%           
=======================================
  Files           ?       71           
  Lines           ?     6461           
  Branches        ?        0           
=======================================
  Hits            ?     4246           
  Misses          ?     1895           
  Partials        ?      320           

Flag	Coverage Δ
unit	65.71% <56.25%> (?)

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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• OWNERS [xhebox]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-11 03:41:58.149821097 +0000 UTC m=+1279071.906956666: ☑️ agreed by xhebox.