observer: read the server version from HTTP status

[yorkhellen]

What problem does this PR solve?

Issue Number: close #456

Problem Summary:
refactor: read the server version from HTTP status response body instead of the initial handshake
What is changed and how it works:
read the server version from HTTP status response body, suck like:

{"connections":0,"version":"8.0.11-TiDB-v8.0.0","git_hash":"8ba1fa452b1ccdbfb85879ea94b9254aabba2916"}

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]

Hi @yorkhellen. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: djshow832

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• OWNERS [djshow832]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-17 06:01:25.697575491 +0000 UTC m=+404304.808621938: ☑️ agreed by djshow832.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.83333% with 7 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@bbfad0b). Click here to learn what that means.

Files	Patch %	Lines
pkg/balance/observer/health_check.go	70.83%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #510   +/-   ##
=======================================
  Coverage        ?   64.86%           
=======================================
  Files           ?       65           
  Lines           ?     6096           
  Branches        ?        0           
=======================================
  Hits            ?     3954           
  Misses          ?     1837           
  Partials        ?      305           

Flag	Coverage Δ
unit	64.86% <70.83%> (?)

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