api: add /debug and /metrics

[djshow832]

What problem does this PR solve?

Issue Number: close #425

Problem Summary:
Currently, we are using /api/debug and /api/metrics, which are inconsistent with other components. Thus, TiDB Operator, TiUP, and Dashboard need to handle it specially.

What is changed and how it works:
Add /debug and /metrics.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

zhangmingdeMacBook-Pro:tiproxy zhangming$ curl http://127.1:3080/debug/pprof/goroutine > g
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  5137    0  5137    0     0   584k      0 --:--:-- --:--:-- --:--:-- 2508k
zhangmingdeMacBook-Pro:tiproxy zhangming$ curl http://127.1:3080/metrics > m
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    44  100    44    0     0   6075      0 --:--:-- --:--:-- --:--:-- 44000
zhangmingdeMacBook-Pro:tiproxy zhangming$ curl http://127.1:3080/api/debug/pprof/goroutine > g
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  4980    0  4980    0     0   563k      0 --:--:-- --:--:-- --:--:-- 2431k
zhangmingdeMacBook-Pro:tiproxy zhangming$ curl http://127.1:3080/api/metrics > m
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    48  100    48    0     0   7313      0 --:--:-- --:--:-- --:--:-- 48000

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]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-12-27 10:22:05.365541091 +0000 UTC m=+1647616.402768047: ☑️ agreed by xhebox.

[djshow832]

/retest

[wuhuizuo]

/retest