observer, net: read packet from SQL port in health check

[djshow832]

What problem does this PR solve?

Issue Number: close #577

Problem Summary:
#510 removes reading one packet from the SQL port, but some TiDB versions don't respond after establishing the TCP connection before finishing loading statistics.

Handshake until TLS is complicated. It needs to consider capabilities like proxy-protocol and TLS. So maybe I'll use another way to detect the validity of TLS certs.

What is changed and how it works:
Read one packet from the SQL port in health checking.

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

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None

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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  Coverage        ?   67.52%           
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  Files           ?       76           
  Lines           ?     6900           
  Branches        ?        0           
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  Hits            ?     4659           
  Misses          ?     1904           
  Partials        ?      337           

Flag	Coverage Δ
unit	67.52% <100.00%> (?)

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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:

• 2024-06-25 04:25:12.759395153 +0000 UTC m=+693639.244883985: ☑️ agreed by xhebox.