metrics, balance: add metrics for load balance

[djshow832]

What problem does this PR solve?

Issue Number: close #555

Problem Summary:
Users may need these metrics for load balance:

• The reasons for connection migration
• Cross-zone traffic

What is changed and how it works:

• Add ConfigGetter to pull configs instead of pushing configs because Potential concurrency bugs in ConfigManager #532
• BackendObserver decides backend locality by TiProxy labels and backend labels. This is moved from FactorLocation so that both FactorLocation and BackendConnMgr can know backend locality by BackendInst
• Add cross-az traffic and connection migration metrics

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 metrics of cross-az traffic and connection migration reasons

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 74.19355% with 16 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@de4a59b). Learn more about missing BASE report.

Files	Patch %	Lines
pkg/balance/router/metrics.go	45.45%	4 Missing and 2 partials ⚠️
pkg/balance/router/router.go	0.00%	5 Missing ⚠️
pkg/balance/router/router_static.go	0.00%	2 Missing ⚠️
pkg/balance/factor/factor_location.go	50.00%	0 Missing and 1 partial ⚠️
pkg/balance/policy/simple_policy.go	83.33%	1 Missing ⚠️
pkg/balance/router/router_score.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #556   +/-   ##
=======================================
  Coverage        ?   67.06%           
=======================================
  Files           ?       76           
  Lines           ?     6926           
  Branches        ?        0           
=======================================
  Hits            ?     4645           
  Misses          ?     1940           
  Partials        ?      341           

Flag	Coverage Δ
unit	67.06% <74.19%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [xhebox]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-06-13 01:45:03.106577465 +0000 UTC m=+601857.159889384: ☑️ agreed by xhebox.