backend, metrics: add metrics for traffic and handshake

[djshow832]

What problem does this PR solve?

Issue Number: close #406

Problem Summary:
We need metrics for traffic and handshake.

What is changed and how it works:
Add metrics for traffic and handshake.

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.

- Add metrics for traffic and handshake.

[djshow832]

/retest

[djshow832]

/retest

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.72289% with 16 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@3d80091). Click here to learn what that means.

Files	Patch %	Lines
pkg/proxy/backend/metrics.go	80.00%	6 Missing and 3 partials ⚠️
pkg/proxy/net/packetio.go	50.00%	4 Missing ⚠️
pkg/proxy/backend/backend_conn_mgr.go	88.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #477   +/-   ##
=======================================
  Coverage        ?   64.24%           
=======================================
  Files           ?       61           
  Lines           ?     5761           
  Branches        ?        0           
=======================================
  Hits            ?     3701           
  Misses          ?     1772           
  Partials        ?      288           

Flag	Coverage Δ
unit	64.24% <80.72%> (?)

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[djshow832]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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• OWNERS [xhebox]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-03-02 13:05:35.886891064 +0000 UTC m=+1312824.634514160: ☑️ agreed by xhebox.