metricsreader: move cluster-scoped metrics lifecycle into backend clusters (#1105)

[ti-chi-bot]

This is an automated cherry-pick of #1105

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

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.73585% with 59 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-nextgen-202603@8aefd9f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/manager/backendcluster/metrics_querier.go	65.51%	27 Missing and 3 partials ⚠️
pkg/manager/backendcluster/cluster.go	64.51%	10 Missing and 1 partial ⚠️
pkg/balance/metricsreader/query_result.go	80.43%	6 Missing and 3 partials ⚠️
pkg/manager/backendcluster/manager.go	55.55%	4 Missing ⚠️
pkg/balance/metricsreader/metrics_reader.go	82.35%	3 Missing ⚠️
pkg/server/server.go	75.00%	2 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##             release-nextgen-202603    #1123   +/-   ##
=========================================================
  Coverage                          ?   67.33%           
=========================================================
  Files                             ?      145           
  Lines                             ?    15263           
  Branches                          ?        0           
=========================================================
  Hits                              ?    10277           
  Misses                            ?     4292           
  Partials                          ?      694           

Flag	Coverage Δ
unit	67.33% <77.73%> (?)

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: YangKeao

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

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Details

Needs approval from an approver in each of these files:

• OWNERS [YangKeao]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-04-13 09:22:04.510715895 +0000 UTC m=+1380129.716075952: ☑️ agreed by YangKeao.