metricsreader: move cluster-scoped metrics lifecycle into backend clusters

[YangKeao]

What problem does this PR solve?

Issue Number: close #1099

• docs: add cluster-scoped multi-cluster runtime design #1102
• config: add backend cluster schema and multi-port listener config #1103
• backendcluster: add cluster manager and cluster-scoped topology runtime #1104
• metricsreader: move cluster-scoped metrics lifecycle into backend clusters #1105 <-
• router: add port-based routing groups #1106
• backendcluster: route PD and backend traffic through cluster DNS #1107

What is changed and how it works:

Move backend metrics lifecycle to the cluster-scoped runtime introduced by the backend-cluster manager.

This PR changes metrics collection so that:

• each backend cluster owns its own metrics reader
• backend metrics election is cluster-scoped
• metrics queries are dispatched through the manager to the right cluster readers

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.

None

[ti-chi-bot]

Skipping CI for Draft Pull Request.
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[ti-chi-bot]

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 69.04762% with 156 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@29264bd). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/manager/backendcluster/manager.go	63.26%	72 Missing and 18 partials ⚠️
pkg/manager/backendcluster/metrics_querier.go	65.51%	27 Missing and 3 partials ⚠️
pkg/server/server.go	61.11%	6 Missing and 1 partial ⚠️
pkg/balance/observer/health_check.go	44.44%	4 Missing and 1 partial ⚠️
pkg/proxy/backend/backend_conn_mgr.go	54.54%	4 Missing and 1 partial ⚠️
pkg/balance/router/router.go	55.55%	4 Missing ⚠️
pkg/balance/router/router_static.go	0.00%	4 Missing ⚠️
pkg/manager/namespace/manager.go	20.00%	4 Missing ⚠️
pkg/balance/metricsreader/metrics_reader.go	78.57%	3 Missing ⚠️
pkg/balance/router/group.go	81.25%	3 Missing ⚠️
... and 1 more

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1105   +/-   ##
=======================================
  Coverage        ?   67.20%           
=======================================
  Files           ?      144           
  Lines           ?    15195           
  Branches        ?        0           
=======================================
  Hits            ?    10212           
  Misses          ?     4285           
  Partials        ?      698           

Flag	Coverage Δ
unit	67.20% <69.04%> (?)

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[YangKeao]

/test all

[YangKeao]

/test all