metricreader: fix data race in BackendReader

[djshow832]

What problem does this PR solve?

Issue Number: close #618

Problem Summary:
After the query results are returned to factors, the results are still updated by the BackendReader, which may cause data race.

What is changed and how it works:
Always generate the query results from scratch and replace the query results all at once so that the query result map won't be updated.

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

Attention: Patch coverage is 80.55556% with 7 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@0a52ff1). Learn more about missing BASE report.

Files	Patch %	Lines
pkg/balance/metricsreader/backend_reader.go	84.84%	3 Missing and 2 partials ⚠️
pkg/balance/metricsreader/query_result.go	33.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #619   +/-   ##
=======================================
  Coverage        ?   68.92%           
=======================================
  Files           ?       86           
  Lines           ?     7956           
  Branches        ?        0           
=======================================
  Hits            ?     5484           
  Misses          ?     2086           
  Partials        ?      386           

Flag	Coverage Δ
unit	68.92% <80.55%> (?)

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-05 02:39:22.919732183 +0000 UTC m=+233292.786831271: ☑️ agreed by xhebox.